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.
adjective suffix, "full of or characterized by," from Old English -ig, from Proto-Germanic *-iga- (source also of Dutch, Danish, German -ig, Gothic -egs), from PIE -(i)ko-, adjectival suffix, cognate with elements in Greek -ikos, Latin -icus (see -ic). Originally added to nouns in Old English; used from 13c. with verbs, and by 15c. even with other adjectives (for example crispy).