The annoying way one person (usually female) will pat another person (usually male) on the back during a hug, signifying that sex between the two will never, EVER happen. However, this can also used by males in a professional setting to avoid premeditated sexual harassment charges by a female colleague.
Ryan: "Dude, I hugged Veronica and she just gave me the back pat AGAIN."
Justin: "Yeah... she did that to me, too. We are so NOT getting any ass from her."