"Chwys" comes from Proto-Indo-European *sweyd- (“to sweat”), which makes it a cognate of this long but not exhaustive list of words: English "sweat", German "Schweiß", Swedish "svett", French "sueur", Spanish "sudor", Persian خوی (xway), Sanskrit स्वेद (svéda), Latvian "sviedri", Tocharian B "syā-", and Albanian "djersë".
I used siwmper, because I have always thought sweatshirt and jumper were the same thing?
It might be a difference in English dialects, perhaps. In Britain it tends to be 'pullover, jumper' for a knitted cotton or woollen top to keep you warm generally, and 'sweat-shirt' as something originally designed for putting on after heavy exercise. But again, usages may vary.