An Etymological Dictionary of the German Language/Annotated/schlau
schlau, adjective, ‘sly, crafty, cunning,’ early Modern High German only, formed from Low German slû; compare Dutch sluw, ‘sly’; akin also probably to Old Icelandic slœ́gr, Middle English sleigh, English sly, which, as Modern High German verschlagen, ‘cunning,’ indicates, is perhaps connected with the root slah, ‘to strike.’ It is uncertain how far these terms are due to earlier loan-words, and whether Old Icelandic slœ́gr is the ultimate source of them all.