Trunks are short close-fitting garments worn to swim in. Swimming trunks, a pair of trunks, wear trunks.
The word is plural and takes a pair of, like trousers and shorts. Swimming trunks is the standard British term for men's swimwear, where American English says swim trunks or just trunks. Boxing trunks are the same shape for a different sport. Note that trunk in the singular means several unrelated things — a tree, a case, an elephant's nose — and none of them is this.
1nountrunksShort close-fitting garments worn to swim in.
“He packed his swimming trunks and a towel for the beach.”
trunks is a garment.