butt yawn 

The first fart or “sphincter stretch” of the morning. While you sleep, gas builds up in your lower intestine and must be expelled when you wake. The "butt yawn" usually takes place during the morning urination or when you first roll out of bed. This fart typically does not smell badly, however it can be very boisterous. Caution should be taken if others are sleeping nearby.

My wife was not happy at all when I woke her up with an extremely loud "butt yawn" this morning.

butt yawn by Nacho Burris November 16, 2016