Meaning of “sly” - Learner’s Dictionary

sly

adjective uk us /slaɪ/ (slyer, slyest)

deceiving people in a clever way to get what you want

sly smile

a smile that shows you know something that other people do not:

"I know why Chris didn't come home yesterday," she said with a sly smile.
slyly adverb

(Definition of “sly” from the Cambridge Learner’s Dictionary © Cambridge University Press)