Crossword Dictionary

SLY

crafty, sly, foxy, wily, artful, cunning, craftier, craftily, craftiest, versute - a 
skillful in deceiving 

wordplays

Synonyms

artful, beguiling, cagey (also cagy), crafty, cunning, cute, designing, devious, dodgy [chiefly British], foxy, guileful, scheming, shrewd, slick, subtle, tricky, wily

merriam-webster

Examples

His eyes too seemed strange; at one moment they looked impudently sly and at the next glanced round in alarm.

Ivanushka, sipping out of her saucer, looked with sly womanish eyes from under her brows at the young men.

A footman, thinking no one saw him, was drinking on the sly what was left in the glasses.

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Etymology

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."

etymonline