"Oh, he's going to have kittens over this when he finds out about it!" Meaning somebody's going to get real upset.
But why kittens? After all, there's nothing cuter, so why associate fuzzy adorableness with a reaction to upsetting news?
|Kittens! (Photo: Public Domain)|
According to the site Cat-ch Phrases, “In medieval times it was believed that if a pregnant woman was in pain, she had been cursed by a witch and had kittens inside clawing at her belly. Witches, they believed, could control cats, and could eliminate the kittens. Since a woman believing she was going to give birth to a litter of kittens would become hysterical, the phrase has, over time, come to mean being in an angry panic.”
Phrases.org tells us this superstition originated in Scotland and was documented as late as 1654 in a court case where a woman accused of trying to get an abortion used as her defense the excuse that she had 'cats in her bellie.'
So there you have it!
Phrases.org: from "How Did It Begin?" by R. Brasch (Pocket Books, New York, 1966)