An Etymological Dictionary of the German Language/Annotated/schließen
schließen, verb, ‘to close, shut, include, infer,’ from Middle High German slieȥen, Old High German slioȥan, ‘to shut,’ Old Saxon *slûtan (equivalent to Middle Low German and Low German slûten), is attested by slutil, ‘key’; Dutch sluiten, ‘to lock up,’ Old Frisian slûta; further Northern English sloat, slot, ‘bolt of a door.’ In Old Icelandic and Gothic the corresponding verbs and derivatives are wanting. The Teutonic root certainly originated in pre-Teutonic sklū̆d — the combination skl is not tolerated in Teutonic, — and hence it may be compared with Latin claudo for *sclaudo (Aryan root klaud, as well as sklaud), as a cognate term. See Schloß and Schlüssel.