adj. 1 cunning, artful, crafty, clever, wily, guileful, underhand(ed), deceitful, treacherous, foxy, scheming, plotting, designing, conniving, furtive, shrewd, sneaky, stealthy, insidious, devious, disingenuous, tricky, shifty, sharp, canny, Colloq shady It was devilishly sly of him to manoeuvre you into paying for his party 2 impish, elfish, roguish, mischievous, puckish, devilish, scampish, naughty, arch, waggish Even as he was flirting with Allison he was giving Antonia sly, conspiratorial winks
n. 3 on the sly. slyly or slily, quietly, surreptitiously, covertly, stealthily, furtively, sneakily, underhandedly, clandestinely, Colloq on the q.t. or Q.T., on the side Although her parents had forbidden her to see him again, Antonia continued to meet Kevin on the sly