Sly

1. Artfully cunning; secretly mischievous; wily.
2. (having a positive sense) Dexterous in performing an action, so as to escape notice

Synonyms: nimble, skillful, cautious, shrewd

3. Done with, and marked by, artful and dexterous secrecy; subtle
4. Light or delicate; slight; thin.

artful

cunning

knowing

sharp

crafty

shrewd

shifty

sly as a fox

slim

wily