From Middle English sly, sley, sleigh, sleiȝ, from Old Norse slægr, slœgr, from Proto-Germanic *slōgiz, from Proto-Indo-European *slak-. Cognate with Icelandic slægur ("crafty, sly"), Norwegian Nynorsk sløg ("sly"). Related to sleight, slay. In all likelihood, however, unrelated with Saterland Frisian slau ("sly, crafty"), Dutch sluw ("sly, cunning"), Low German slu ("sly, cunning"), German schlau ("clever, crafty").
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