Wiktionary
warmup jacket
n. A jacket with an elasticated waist and a zipper up the front, worn by athletes while training; worn by others as a fashion accessory, usually carrying a team logo.
Usage examples of "warmup jacket".
As they went, Andy noticed a man in a baseball warmup jacket running across to the house they had just left, and he felt a tickle of unease.
She says she saw the man in the Celtics warmup jacket that night of the fifteenth, too, only it was about eight-thirty.