sly

[1]	on the sly , slyly/slily , quietly , surreptitiously , covertly , stealthily , furtively , sneakily , underhandedly , clandestinely , (Colloq) on the q.t.or Q.T , on the side

adj.

[1]	crafty , cunning , artful , cunning , dodgy , crafty , clever , foxy , guileful , wily , guileful , knavish , slick , underhand(ed) , deceitful , tricksy , tricky , treacherous , foxy , wily , scheming , plotting , designing , conniving , furtive , shrewd , sneaky , stealthy , insidious , devious , disingenuous , tricky , shifty , sharp , canny , (Colloq) shady
[2]	impish , elfish , roguish , mischievous , puckish , devilish , scampish , naughty , arch , waggish