Britain calls it a jumper. America calls it a sweater. Neither word has anything to do with the thing itself being warm and knitted — and one of them has a genuinely surprising origin.
“Jumper” has nothing to do with jumping
The obvious guess is that a jumper is called that because you jump into it, pulling it over your head. That is not where the word comes from. It traces back to a much older dialect word, jump or jup, describing a man’s short coat or a woman’s under-bodice — a word borrowed, in turn, from the French jupe, meaning a short coat or tunic. It surfaces in English in the mid-nineteenth century, describing a loose outer garment reaching to the hips, and only later settles into meaning the knitted pullover we would recognise today.
Source: World Wide Words
“Sweater” is exactly as literal as it sounds
The American term has a far more straightforward story. It comes directly from the verb to sweat — the garment was originally associated with heavy wool worn specifically to work up a sweat during exercise, and the word caught on alongside the growth of organised sport in the late nineteenth century. Where “jumper” arrived by way of French tailoring terms and several centuries of drift, “sweater” describes exactly what it says on the label.
Source: Wikipedia
Why British English kept the older word
This is a small, neat example of a wider pattern in the two dialects. British English quite often preserves an older term borrowed centuries earlier, while American English adopts a newer, more literal one that arrives alongside a cultural shift — in this case, the rise of organised sport and sportswear in the US around the turn of the twentieth century. Neither word is more correct. They simply arrived by different routes, at different times, from different sources.
And the rest of the world?
Ireland and Australia mostly follow the British “jumper.” Elsewhere, “pullover” and “jersey” both turn up as alternatives depending on the country and the exact style of garment, which is why a product listing aimed at an international audience will sometimes use two or three of these words in the same sentence, almost as a hedge.
Whichever word you use, the same buying advice applies regardless of what you call it. Start with how to choose an ugly Christmas jumper.