1510s, "lazy man," variant of slouk (1560s), probably from a Scandinavian source, perhaps Old Norse slokr "lazy fellow," and related to slack (adj.) on the notion of "sagging, drooping." Meaning "stooping of the head and shoulders" first recorded 1725. Slouch hat, made of soft material, first attested 1764.
"walk with a slouch," 1754; "have a downcast or stooped aspect," 1755; from slouch (n.). Related: Slouched; slouching (1610s as a past-participle adjective; 1660s of persons, 1690s of hats).