Really? If you're wearing the right jumper you ain't sweating while you jump. You're just kinda comfy.
I have no idea why we say jumper for a woollen garment for the upper body. But to call it a sweater implies that it's overdoing its intended function. Jumper implies the freedom to, well, jump, with no suggestion of discomfort. Sweating in a woollen garment can be unpleasant IME.
The expression that seems arse-about in the US to me is "I could care less". Here we say I couldn't care less, which actually makes sense. But then it could well be that making sense is over-rated. I think maturity is over-rated.