surreptitious - sneaky
The term ‘surreptitious’ refers to action or behaviour done sneakily in an attempt to prevent anyone from seeing or knowing about it, typically to avoid trouble or to trick somebody. It is generally characterized by a skillful avoidance of detection and a violation of trust, a custom, law, or authority.
The executive received a number of surreptitious payments to make sure the deal went through.
The surreptitiousness of the group's actions led the police to believe that they were engaging in criminal activity.
I did not want my boss to think I was bored during the meeting, so I surreptitiously glanced at my watch to see how long was left.