This is usually a way to attempt to control an unexpected erection, and is used to avoid pitching a tent.
This is in reference to a baby kangaroo (a joey) peeking out from his mother's pouch.
--"Whoa! That was super-sexy! Let's go on a beer run!"
"Dude, I totally can't get up from the couch just now...I'm Rooing! Flip over to Anderson Cooper 360 for like two minutes and i'll be good to go!"
--"Right on! Just don't lift your shirt--I don't want to see that Joey!"