Sly
Middle English (also in the sense ‘dexterous’): from Old Norse slœgr ‘cunning’, originally ‘able to strike’ from the verb slá ; compare with sleight.
wiktionary
From Middle English sly, sley, sleigh, sleiȝ, from Old Norse slægr, slœgr(“sly, cunning”, literally “capable of hitting or striking”), from Proto-Germanic *slōgiz(“lively, agile, cunning, sly, striking”), from Proto-Indo-European *slak-(“to hit, throw”). 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”).
etymonline
sly (adj.)
c. 1200, "skillful, clever, dexterous," from Old Norse sloegr "cunning, crafty, sly," from Proto-Germanic *slogis (source also of Low German slu "cunning, sly," German schlau), probably from base *slak- "to strike, hit" (see slay (v.)), with an original notion of "able to hit." Compare German verschlagen "cunning, crafty, sly," schlagfertig "quick-witted," literally "strike-ready," from schlagen "to strike." A non-pejorative use of the word lingered in northern English dialect until 20c. On the sly "in secret" is recorded from 1812. Sly-boots "a seeming Silly, but subtil Fellow" is in the 1700 "Dictionary of the Canting Crew."
