1580s, "to strut in a defiant or insolent manner;" earliest recorded usages are in Shakespeare ("Midsummer Night's Dream," "2 Henry IV," "King Lear"), probably a frequentative form of swag (v.) "to sway." Meaning "to boast or brag" is from 1590s. Related: Swaggered; swaggering. The noun is attested from 1725.