This term originates in Knoxville, Tn by way of Jacksboro, Tn. It refers to getting caught masturbating in the morning by your mother when she is bringing you lunch money.
Josh got Mammy Johnson'd this morning before work. I bet he's gonna have an awkwarddinner.
When you fail at something in life (eg. failing an exam or not getting a job), go home and spend an hour just standing in the shower, propping yourself up against the wall with your eyes on your feet.
Caleb knew the only thing to do after he didnt get prefect was take a failure shower.