late 14c., late 13c. as a surname, "habitually lazy person," from Middle English sluggi "sluggish, indolent," probably from a Scandinavian word; cf. dialectal Norwegian slugga "be sluggish," dialectal Norwegian sluggje "heavy, slow person," dialectal Swedish slogga "to be slow or sluggish." Adjective sluggy is attested in English from early 13c.
As an adjective meaning "sluggish, lazy" from 1590s. Related: Sluggardly.