warning tap 

knocking on the door before you barge in on roommate having sex

I gave him a warning tap; unfortunately, the door was unlocked.

warning tap by word analyst May 7, 2016

warning tap 

The universal tap given (usually on a thigh), signaling an impending ejaculation.

He was such a gentalman. He give me a warning tap when I was on top.

warning tap by Chg0 January 20, 2016