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.
Phrases and idioms:
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.
Derivatives:
slyly adv. (also slily). slyness n.
Etymology: ME sleh etc. f. ON sloegr cunning, orig. 'able to strike' f. sloacuteg- past stem of slaacute strike: cf. SLEIGHT
Useful english dictionary. 2012.