Is it ensure, insure, or assure?
late 15c., "immoral woman," later (16c.) also "rascal, knave" (regardless of gender); probably from a continental Germanic source, cf. 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 *sleubh- "to slide, slip" (see sleeve). Meaning "person careless of dress or negligent of cleanliness" is from 1520s. Cf. also slut.