adj. (slyer, slyest) 1 cunning; crafty; wily. 2 a (of a person) practising secrecy or stealth. b (of an action etc.) done etc. in secret. 3 hypocritical; ironical. 4 knowing; arch; bantering; insinuating. 5 Austral. & NZ sl. (esp. of liquor) illicit. on the sly privately; covertly; without publicity (smuggled some through on the sly). sly dog colloq. a person who is discreet about mistakes or pleasures. slyly adv. (also slily). slyness n.
[ ME sleh etc. f. ON sloegr cunning, orig. 'able to strike' f. slóg- past stem of sl strike: cf. SLEIGHT ]