|Wait. Is that a.... spelling mistake? Why, yes, it is!|
I recently saw someone (no doubt a Canadian) write "rigour mortis" instead of the correct "rigor mortis".
I suspect a Canadian because we Canadians have a very strong tendency to write "-or" words as "-our" just to prove we're not American -- even when "-or" is the correct spelling. See these posts:
and for the explanation of the :-or/-our" split, see this post:
"Rigour" derives ultimately from the Latin rigor (unbending quality, stiffness, rigidity), but it came into English in the 1400s by way of Middle French, where rigor had morphed into rigour. British English and other varieties of Commonwealth English retained this spelling, whereas American English opted for "rigor".
"Rigor mortis", on the other hand, was borrowed directly from Latin in the early 19th century. It retains its Latin spelling in all varieties of English.
And to make life more complicated, the adjective "rigorous" is also so spelled in all varieties of English.
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