noun
trunks (swimming, boxing, etc.)
Refers to sports trunks, especially for swimming or boxing. Not used for casual shorts.
彼は新しい水泳用のトランクスを買った。
He bought new swimming trunks.
ボクシングの試合では、選手はトランクスを着
In a boxing match, the fighters wear trunks.
noun
boxer shorts; boxers; trunks (underwear)
Refers to men's underwear, specifically loose-fitting boxer shorts. In Japanese, トランクス is the common term for boxers, while ブリーフ is used for briefs.
彼はトランクスをはいて寝ている。
He sleeps in boxer shorts.
このトランクスは綿100%で肌触りがいい。
These boxers are 100% cotton and feel nice against the skin.