A nod and a wink
Definition 1 of 1
Pro Tip 1/3Secretive Communication
Used when someone wants to communicate secretly or show agreement without using explicit words. 
The committee chair gave me a nod and a wink, signaling I should stay quiet about the decision.
Pro Tip 2/3Implicit Understanding
Implies that both parties understand the hidden meaning without needing further explanation. 
I mentioned the surprise party with a nod and a wink; everyone knew to keep it a secret.
Pro Tip 3/3Non-verbal Cue
Can be used to describe an actual gesture or used figuratively to describe a subtle sign. 
After a nod and a wink from the coach, the player faked a left turn and passed the ball.