An Etymological Dictionary of the German Language/schlürfen
schlürfen, vb., ‘to sip, lap, drink,’ ModHG. only; probably, however, its non-occurrence in earlier HG. is only an accident (MidHG. *slürfen, OHG. *slurfen); according to the HG. permutation Du. slurpen, ‘to sip,’ is allied. The stem is not found elsewhere; its origin is obscure.