An Etymological Dictionary of the German Language/Annotated/Schloß
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Schloß, neuter, ‘lock, clasp; castle, palace,’ from Middle High German slō̆ȥ, neuter, ‘bolt, band, lock, fetter, castle, citadel,’ Old High German slŏȥ, neuter, ‘lock, bolt’; corresponding to Northern English slot, sloat, ‘bolt, crossbar’; allied to schließen.