sly (SECRETIVE)
adjective 
avoiding letting other people know what you are thinking or intending to do or have done 
"You'll find out eventually," said Mary with a sly smile.
He's a sly one! He kept his inheritance secret for ten years.
When he referred to people not paying their fair share, I think he was having a sly (=not openly expressed) dig at me.

(Australian informal) Sly grog is illegally sold alcoholic drink. 

sly
noun [U] 
If you do something on the sly you do it secretly because you should not be doing it. 
He drives his mother's car on the sly while she's out at work.

slyly
adverb 
She grinned slyly and refused to tell me where the money came from.

slyness
noun [U]