Comment by ASalazarMX

That's citing the pocket trivia book How Did It Begin? Customs, Superstitions and Their Romantic Origins by Rudolph Brasch. As he cites "a superstition, once prevalent among Scottish people" he likely based his claims on Notes on the Folk-Lore of the North-East of Scotland by Rev. Walter Gregor. This reads: "It was a notion that a male cat, when he jumped, emitted urine, and at times semen. Hence great care was used to keep male cats at a distance from food, for another notion was, that, if a cat did chance to jump over food, the one, who was unfortunate enough to partake of it, conceived cats."

This doesn't seem to match the use of the idiom. Meanwhile Greens Dictionary of Slang suggests the meaning is related to the nervousness of pregnant cats. https://greensdictofslang.com/entry/42jrf3i#nizqrgq