One commonly cited explanation for why we call potatoes spuds goes like this: A 19th century activist group called The Society for the Prevention of an Unwholesome Diet, or SPUD, was formed to keep potatoes out of Britain. This group didn't want anyone eating the tubers. The story was perpetuated in Mario Pei's 1949 book, The Story of Language.
But it's clear that Pei was wrong about where the nickname originated, for one very good reason: Previous to the mid-20th century, abbreviations were prevalent in text, but pronouncing them as words was not something people typically did—that's a very modern phenomenon. In fact, according to linguist David Wilton, “There is only one known pre-20th-century [English] word with an acronymic origin, and it was in vogue for only a short time in 1886. The word is 'colinderies' or 'colinda,' an acronym for the Colonial and Indian Exposition held in London in that year.” (No surprise, then, that the word “acronym” didn’t pop up until 1943.)
The explanation for why we sometimes refer to potatoes as spuds is much simpler.
Among other definitions, a spud is a sharp, narrow spade used to dig up large-rooted plants. Around the mid-19th century—the first documented reference occurs in 1845 in New Zealand—this implement of destruction began lending its name to one of the things it was often used to dig up: potatoes. Eventually, the nickname caught on throughout the English-speaking world.
The ultimate origin of the word “spud” isn’t known. It first appeared in English around 1440 and referred to a short dagger, possibly from the Dutch spyd, the Old Norse spjot (spear), or the Latin spad (sword). Whatever the case, after the 15th century, the meaning of the word expanded: Instead of referring just to “a short dagger,” a spud could be one of various types of digging implements—and, eventually, referred to those tubers we all know and love.
Interestingly, when potatoes were first introduced to Europe, they met with a lot of resistance for a variety of reasons. Some people thought they were poisonous (before wild potatoes were domesticated, they actually were poisonous—and sprouts still are), while others refused to eat them because they weren’t mentioned anywhere in the Bible.