cunning or wily

sly as a fox.

stealthy, insidious, or secret.

playfully artful, mischievous, or roguish

sly humor.

on the sly

secretly; furtively

a tryst on the sly.

adjective

honest , open , simple , straight , straightforward , unclever

adjective

arch , artful , astute , bluffing , cagey , calculating , canny , captious , conniving , covert , crafty , crooked , cunning , deceitful , deceptive , delusive , designing , dishonest , dishonorable , dissembling , double-dealing , elusive , foxy , furtive , guileful , illusory , impish , ingenious , insidious , intriguing , mean , mischievous , plotting , roguish , scheming , secret , sharp , shifty , shrewd , slick , smart , smooth , sneaking , stealthy , subtle , traitorous , treacherous , tricky , underhand , unscrupulous , wily , secretive , sneaky , surreptitious , cautious , clandestine , clever , devious , diplomatic , disingenuous , duplicitous , evasive , knowing , nimble , serpentine , sinuous , skillful , strategic , underhanded , wary , weaselly