late 15c., "immoral woman," later (16c.) also "rascal, knave" (regardless of gender); probably from a continental Germanic source, compare Middle Flemish sloovin "a scold," sloef "untidy, shabby," Dutch slof "careless, negligent," Middle Low German sloven "put on clothes carelessly," from Proto-Germanic *slaubjan, from PIE root *sleubh- "to slide, slip." Meaning "person careless of dress or negligent of cleanliness" is from 1520s. Also see slut.
suffix forming adjectives from nouns and meaning "having qualities of, of the form or nature of" (manly, lordly), "appropriate to, fitting, suited to" (bodily, earthly, daily); irregularly descended from Old English -lic, from Proto-Germanic *-liko- (Old Frisian -lik, Dutch -lijk, Old High German -lih, German -lich, Old Norse -ligr), related to *likom- "appearance, form" (Old English lich "corpse, body;" see lich, which is a cognate; see also like (adj.), with which it is identical).
<a href="https://www.etymonline.com/word/slovenly">Etymology of slovenly by etymonline</a>
Harper, D. (n.d.). Etymology of slovenly. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/slovenly