1550s, "trim, neat, spruce, smart," possibly an alteration of Low German smuk "trim, neat," from Middle Low German smücken "to adorn" (originally "to dress," secondary sense of words meaning "to creep or slip into"), from the same source as smock. The meaning "having a self-satisfied air" is from 1701, an extension of the sense of "smooth, sleek" (1580s), which was commonly used of attractive women and girls. Related: Smugly; smugness.