Do you know ghouls from goblins and ghosts?
early 14c., "quiet, modest, demure," from Old French coi, earlier quei "quiet, still, placid, gentle," ultimately from Latin quietus "resting, at rest" (see quiet (n.)). Meaning "shy" emerged late 14c. Meaning "unwilling to commit" is 1961. Related: Coyly; coyness.