Girl Wearing A Christmas Sweater

A particularly loud, colorful, or decorative sweater has been known in America the past twenty years as a "Cosby Sweater", being named after comedian Bill Cosby's penchant for wearing them during the run of The Cosby Show.
Customary wear
Thin sweaters may be worn tucked in to the waistband of trousers; but otherwise, men's sweaters are to be worn untucked. Nonetheless, some individuals, including some television and film actors, have been known for wearing tucked-in sweaters.