Old English clænsunge "cleansing, purifying, castigation; chastity, purity," verbal noun from the root of cleanse. As a present participle adjective, attested from c.1300.
Old English clænsian "to cleanse, purge, purify, chasten, justify," from West Germanic *klainson, from *klainoz (see clean (adj.)). Despite its modern spelling (16c.), it retains its Middle English pronunciation. Related: Cleansed; cleansing.