Dear 100 Hour Board,
Why is it called "in heat" when cats are fertile?
Dear Mr. Henderson,
The OED defines the word "heat," as in "in heat," as "sexual excitement in animals, especially in the female, during the breeding season." As far as the origin of the phrase, Alta suggested it's because a lot of animals go into estrus during spring/summer, and I think she's on to something; Several sources cite the warmer seasons as an explanation for the phrase. However, I'm not sure that's the only explanation, especially since in this early use of the phrase cited by the OED the writer says "The female is in heat in the winter," referring, oddly enough, to an otter. Cats are in heat during the spring and summer, but we also use the phrase for other animals who aren't.
So I think it might also have to do with the association we have between heat and excitement. More excitement means more energy; more energy means more heat. We even talk about electrons being excited by heat. Likewise, in the 1828 Webster's Dictionary, one definition of "cool" is "to lose the heat of excitement or passion," so I think there's a lot of evidence that we associate heat and excitement and have for a long time.
Honestly, the etymology I'm more interested in is the word estrus. I mean, it comes from the Greek word for gadfly—that seems like more of a stretch than heat to me.
'Cause that's when they get all hot and bothered.