Ever wondered why the pronoun I is spelled with a capital letter in English? None of the other first person pronouns (like me, my, or mine) takes a capital letter, nor do the French je, Spanish yo, or Italian io, and nor did the Old English equivalent, ic. So where did the capital I come from? Well, upper case I didn’t begin appearing in the language until the Middle English period, around 700-800 years ago, by which time the Old English ic had been shortened (in some contexts at least) to i. As scribes and scholars soon discovered, however, this single lower case i was difficult to keep track of in written language and could easily go unnoticed or be dismissed as a dash or line, or even a mistake. As a result, a trend emerged to capitalize I, ensuring that it stood out and wasn’t overlooked.

The pronoun I is now one of the most commonly used words in the English language (you can expect it to account for around 1 out of every 200 words you use), which in turn helps to make the letter I one of our most commonly used letters: on average, it accounts for around 7 percent of all written language, and thanks to common prefixes like in–, im– and inter–, accounts for over 4% of all the words in a standard dictionary—including the 40 indispensable I-words illustrated here.

1. IATROPISTIA

As well as being the root of words like psychiatry and pediatrics, iatrosthe Greek word for a doctor or healer, is also the origin of a clutch of doctor-related words like iatragenic (an adjective describing a medical condition unintentionally caused or worsened by a doctor), iatramelia (medical negligence), and iatropistia (a lack of faith in doctors). Similarly…

2. IATROPHOBIA

…is the fear of doctors. And if you hate being seen by a doctor you don’t know, then you have xeniatrophobia.

3. ICELET

A drop of water that drips of an icicle? That’s an icelet. And icicles themselves were once known as...

4. ICESHOGGLES

…as well as iceshockles, icybells, and aquabobs.

5. IDLE-WORMS

In Tudor England, there was an old superstition that claimed that worms grew in the fingertips of lazy, unindustrious women—Shakespeare mentions a “round little worm, pricked from the lazy finger of a maid” in Romeo & Juliet. These worms were known as idle-worms, and were used proverbially as a warning against inactivity or indolence. To be sick of the idles, likewise, meant to be bored or lazy.

6. IDLEBACK

An 18th century word for a lazy good-for-nothing. In the sense of something that only serves to annoy, idleback is also another word for a hangnail.

7. IFFING-AND-BUTTING

An old English dialect word for a poor excuse, or unwilling, prevaricating hesitancy. The modern equivalent “ifs or buts,” or “ifs, ands, or buts” has its origins in the 17th century.

8. IGNIFLUOUS

Anything ignifluous is flowing with fire. An igneduct is a pipe used to carry or direct fire or molten material.

9. IGSY-PIGSY

Probably derived from higgledy-piggledy, an igsy-pigsy is a confused muddle or mixture.

10. ILKA

Derived from the same root as ilk, the ancient English word ilka is another word for “each” or “every.” Ilkabody, ultimately, is literally “everybody,” while ilkabody’s-body is an old dialect word for someone or something that is universally popular.

11. ILL-BLENDED

An 18th century euphemism for being in a bad mood.

12. ILL-HEADED

An old 18th century Scots dialect word for someone who is unruly or unmanageable. Heady itself means “headstrong” or “intractable.”

13. ILL-SNORED

Derived from an old Scandinavian word meaning “offended” or “displeased,” if you’re ill-snored then you’re bad tempered.

14. ILLABORATE

The word elaborate is an etymological cousin of labor, and literally means something along the lines of “born out of hard work,” or “appearing to be the result of much effort.” Its long-forgotten opposite is illaborate, an early 17th century word describing anything that looks unfinished, hastily completed, or in need of more work.

15. ILLACHRYMABLENESS

If you’re illachrymable then you’re literally unable to cry, while illachrymableness is another word for mercilessness or pitilessness.

16. IMBERBIC

A word you never knew you needed—a 17th century adjective describing a man who doesn’t have a beard.

17. IMBRANGLE

An imbranglement is a knotty situation or predicament, and so to imbrangle someone is to utterly confuse or entangle them.

18. IMMORIGEROUS

Morigerous, derived from a Latin word for “compliance” or “obedience,” is an old-fashioned word describing anyone who is submissive or agreeable. If you’re immorigerous, ultimately, then you’re obstinate or disobedient.

19. IMPENDIOUSNESS

A 17th century word for lavish, immoderate spending.

20. INBANK

To move inbank is to move downhill or downwards to the ground.

21. INCHMEAL

To do something inchmeal is to do it bit by bit, or part by part.

22. INDEX-LEARNING

Flicking through a book or only perusing its index in order to pick up a basic knowledge of a subject is called index-learning.

23. INDUMIOUS

A word describing extremely rough or stormy weather.

24. INEXPRESSIBLES

Euphemistic Victorian slang for trousers or underwear.

25. INGANGER

Inganging—or in other words, a “going-in”—is an old Yorkshire dialect word for the entrance to a home or building, which makes ingangers visitors or, literally, the people who are coming in your door.

26. INGLEGLEED

An old Scots dialect word for the light of a fire.

27. INKHORNISM

An inkhorn is a small container (originally made out of animal horn) for holding writing ink, but because such equipment was once associated with scholars and academics, smelling of the inkhorn once meant “being excessively pedantic with language or grammar,” while an inkhorn term or inkhorn word is a particularly bookish or obscure word or turn of phrase. Inkhornism is language that uses precisely those kinds of words, while an inkhornist is someone with very pedantic or conservative opinions on language.

28. INKLE-WEAVER

Inkle is another old word for an inkling, which makes an inkle-weaver someone who comes up with ideas. To be as thick as inkle-weavers is an old English expression referring to two very close friends or companions—or two close conspirators who seem up to no good.

29. INKLETH

An inkleth is an inkling—in other words, the slightest hint of an idea.

30. INNERLY

The adjective innerly was originally used to refer to sheltered, unexposed land in the middle of a given plot or district, which would typically be the most fertile or most suitable for building or grazing on. As a result, it came to be used figuratively to mean “friendly,” “sociable” or “of a neighborly disposition,” while if you’re innerly-hearted, then you’re particularly empathetic.

31. INSINUENDO

A portmanteau of insinuation and innuendo. Its entry in the Oxford English Dictionary calls it “a tasteless word.”

32. INSOMNOLENCE

Also known as insomnolency or insomniousness—all formal word for sleeplessness.

33. INSPERGE

To sprinkle or scatter something.

34. INSPISSATION

When you thicken a sauce or some similar concoction by boiling it to evaporate some of its excess liquid, that process is called inspissation. To inspissate is to thicken on congeal.

35. INTERFENESTRATION

The word interfenestration can be used to refer either to the layout of windows in a building, or to the space or area of wall between two windows. If something is interfenestral then it’s located between two windows, while…

36. INTERLACUSTRINE

…anywhere that’s interlacustrine is located between two lakes.

37. INTREASURED

A Shakespearean invention meaning “stored as safely as treasure.”

38. IRROBORATE

An etymological cousin of corroborate, if you irroborate something then you make it stronger.

39. ISABELLINE

Isabelline or Isabel is name of a pale greyish-yellow color, often used to describe the color of horses or birds. According to legend, it takes its name from Isabella Clara Eugenia, the daughter of King Philip II of Spain, who together with her husband Archduke Albert VII of Austria ruled the Spanish Netherlands in the early 1600s. When the city of Ostend (in modern-day Belgium) was besieged in 1601, to show how quickly she presumed her husband and his army could end the standoff Isabella reportedly refused to change her undergarments until the siege was over. Unfortunately for her, the siege went on to last for another three years, so by the time Ostend finally surrendered to the Spanish and Isabella changed her underwear, they were—well, not exactly white.

40. ITERABLE

If iterate means “to do or say something again,” then something that is iterable is capable of being repeated.