Ammon Shea spent a year working his way through the Oxford English Dictionary. The result is his book Reading the OED: One Man, One Year, 21,730 Pages. But in the run-up to his book's publication, Shea shared many of his most bizarre finds in an Oxford University Press blog. Here's an intro:
Absurd Entries is the name that I gave to a certain class of definition that I would come across every so often when reading the OED. They are rarer than the mistakes, and considerably more fun to read. These are the extremely rare moments when the OED does something that is so inexplicable that you have to close the book and check the cover to make sure that it is indeed the same book that you thought. I have decided, without giving too much thought to the matter, to divide them into two separate categories: 'Blatant Disregard for the Reader's Level of Education' and 'What Were They Thinking?'
In the category of Blatant Disregard, the past editors of the OED had seemingly come to the conclusion that since they sat around all day reading about words, accruing a monstrous knowledge of vocabulary, their readers must have done the same, and therefore it was not necessary to talk down to anyone with the definition. For instance trondhjemite is defined as 'Any leucocratic tonalite, esp. one in which the plagioclase is oligoclase'. I have my doubts as to whether anyone has ever thought to themselves 'I wonder what trondhjemite means?' But if someone did, and went to look it up in the OED, it seems unlikely that this definition would clear things up much.
In a similar vein, self-feeling is defined as 'used to render coenaethesis', and occupatio is simply 'preterition'. (I should add that in the online version both coenaethesis and preterition are linked to definitions, which feels a bit like cheating) The word syllogism has a fairly simple and informative definition, and the OED even thoughtfully provides an example of a syllogism at the end of it. Which is written entirely in Latin. Although for sheer incomprehensibility, I do not think that I have seen many entries that can beat the masterful one that was created for the curious word disghibelline - 'To distinguish, as a Guelph from a Ghibelline.'
See also: _floss contributor A.J. Jacobs, who read the Encyclopaedia Britannica and chronicled his experiences in The Know-It-All: One Man's Humble Quest to Become the Smartest Person in the World.