A ruffian; a swaggerer. “From swashing,” says Fuller, “and making a noise on the buckler.” The sword-players used to “swash” or tap their shield, as fencers tap their foot upon the ground when they attack. (Worthies of England.) (A.D. 1662.) (See Swinge-Buckler.)
“A bravo, a swashbuckler, one that for money and good cheere will follow any man to defend him; but if any danger come, he runs away the first, and leaves him in the lurch.” —Florio.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894