sly 
(slī)
(slī)adj. sli·er (slīər), sli·est (slīĕst) also sly·er or sly·est
Idiom: 1. Clever or cunning, especially in the practice of deceit.
2. Stealthy or surreptitious: took a sly look at the letter on the table.
3. Playfully mischievous: a sly laugh.
on the sly
In a way intended to escape notice: took extra payments on the sly.
[Middle English sleigh, from Old Norse slœgr.]
slyly adv.
slyness n.