Wiktionary
n. A knitted cap, such as the one worn by a seaman when on watch.
WordNet
n. a knitted dark blue wool cap worn by seamen in cold or stormy weather
Usage examples of "watch cap".
First me, twisted under my hump, the watch cap popping its bald shock, and then the seconds for him to register the shape of Arturo and the way he moved, and, most important, which direction he was heading.
There was a black watch cap on her head again, pulled down to her ears to hide her light-colored hair.
He pulled a watch cap down over his ears and set out into the night.
Merrill's double-breasted and oddly youthful Navy pea jacket-and, on the peg next to it, his seaman's watch cap.
Claire, a short and plump, but attractive, middle-aged brunette took off her woolen watch cap and mittens before kissing Duke.
Luke saw a young man of his size, but he was wearing a duffel coat and a wool watch cap: he might not have a spare suit in his haversack.
Instead, she was dressed in a slim, black jumpsuit, a black watch cap rolled up above her ears, and snug black boots.