An Etymological Dictionary of the German Language/schließen
schließen, vb., ‘to close, shut, include, infer,’ from MidHG. slieȥen, OHG. slioȥan, ‘to shut,’ OSax. *slûtan (equiv. to MidLG. and LG. slûten), is attested by slutil, ‘key’; Du. sluiten, ‘to lock up,’ OFris. slûta; further Northern E. sloat, slot, ‘bolt of a door.’ In OIc. and Goth. the corresponding vbs. and derivs. are wanting. The Teut. root certainly originated in pre-Teut. sklū̆d — the combination skl is not tolerated in Teut., — and hence it may be compared with Lat. claudo for *sclaudo (Aryan root klaud, as well as sklaud), as a cognate term. See Schloß and Schlüssel.