Merriam-Webster Collegiate Dictionary - sly

 adjective  (slier or ~er; sliest or ~est)  Etymology: Middle English sleighe, sli, from Old Norse sl?gr; akin to Old English slean to strike — more at slay  Date: 13th century  1. chiefly dialect  a. wise in practical affairs  b. displaying cleverness ; ingenious  2.  a. clever in concealing one's aims or ends ; furtive the ~ fox  b. lacking in straightforwardness and candor ; dissembling a ~ scheme  3. lightly mischievous ; roguish a ~ jest a ~ smile  • ~ly also slily adverb  • ~ness noun Synonyms:  ~, cunning, crafty, wily, tricky, foxy, artful, slick mean attaining or seeking to attain one's ends by guileful or devious means. ~ implies furtiveness, lack of candor, and skill in concealing one's aims and methods a ~ corporate raider. cunning suggests the inventive use of sometimes limited intelligence in overreaching or circumventing the cunning fox avoided the trap. crafty implies cleverness and subtlety of method a crafty lefthander. wily implies skill and deception in maneuvering the wily fugitive escaped the posse. tricky is more likely to suggest shiftiness and unreliability than skill in deception and maneuvering a tricky political operative. foxy implies a shrewd and wary craftiness usually involving devious dealing a foxy publicity man planting stories. artful implies indirectness in dealing and often connotes sophistication or cleverness elicited the information by artful questioning. slick emphasizes smoothness and guile slick operators selling time-sharing. SLYBOOTS  noun plural but singular in construction  Date: circa 1700 a sly tricky person; especially one who is cunning or mischievous in an engaging way