One way to hold this word: a rail of clothes chosen for the body that is moving — soft cloth, room to stretch, nothing stiff. Everything the word covers grows out of that rail. First the clothes you train in. Then the same clothes worn simply because they are comfortable. Then, in American shops, the whole part of the store where the un-formal clothes live.
From sports + the suffix -wear, the same -wear as in swimwear, footwear and knitwear: a word that names a whole class of clothing by the occasion it is for.
由 sports 加上字尾 -wear 組成，跟 swimwear、footwear、knitwear 是同一個 -wear。這個字尾的工作就是「用場合來替一整類衣物命名」，所以 sportswear 從一開始就是總稱，不指單一件衣服。
Wiktionary entry supplied in this batch's evidence packet (source_ref kaikki:3f6f69a190036ae4:1370166974)
One branch
為活動與放鬆而做的衣服
這一族就是核心畫面本身，以及它往外一步的結果：同一批衣服，先是為了運動，後來變成「不必正式」的代名詞。
noun · B1
Clothes made for playing sports or exercising, such as tracksuits, shorts and running tops. People also wear these clothes when they are relaxing and want to be comfortable rather than dressed up.
運動服