yawn stick
1.  The act of putting your finger or fingers into someone's mouth while they are yawning, usually as a means of disrupting their yawn and robbing them of all satisfaction.

2.  A vicious game that involves sticking your finger or fingers into people's mouths when they yawn.
"Oh shit!  I just gave her a killer yawn stick!"

"Dammit, I just got yawn stuck!"

"Dammit, I told you, I don't play Yawn Stick!"