adj. 25B6; adjective
she's rather sly : CUNNING, crafty, clever, wily, artful, guileful, tricky, scheming, devious, deceitful, duplicitous, dishonest, underhand, sneaky; archaic subtle.
a sly grin : ROGUISH, mischievous, impish, playful, wicked, arch, knowing.
she took a sly sip of water : SURREPTITIOUS, furtive, stealthy, covert.
25A0; on the sly IN SECRET, secretly, furtively, surreptitiously, covertly, clandestinely, on the quiet, behind someone's back.