sly Definition

sly
adj slyer, slyest

1. Said of people: clever; cunning or wily.

Thesaurus: cunning, shrewd, wily, crafty, tricky, scheming, canny.
2. Surreptitious; secretively deceitful or dishonest.

Thesaurus: secretive, furtive, evasive, stealthy, deceitful, cagey, double-crossing; Antonym: candid, honest.
3. Playfully mischievous.

Example: a sly smile

Derivative: slyly
adverb

Derivative: slyness
noun

Idiom: on the sly

colloq
Secretly or furtively; surreptitiously.

Etymology: 12c: from Norse slægr.