1630s, "a woman negligent or disordered in her dress or household," a word of uncertain origin, probably related to Low German Slattje, Dutch slodder, dialectal Swedish slata "slut" (in the older, non-sexual sense; compare slut). Also compare dialectal English verb slatter "to spill or splash awkwardly, to waste," used of women or girls considered untidy or slovenly. With intrusive -n, perhaps as in bittern.
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/slatternly">Etymology of slatternly by etymonline</a>
Harper, D. (n.d.). Etymology of slatternly. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/slatternly