"Oh Look at her...thats a real mum bum..." or "Look at her she aint got no junk in that trunk, she got mum bum"....
well look at the bums of some moms and you'll see what we mean.... it's very obvious!!(looks like you've spent the last 3 days laying on your back and your ass is flattened...)
The act of being overly nice and polite to the parents of friends/acquaintances, especially mothers, when you are in fact brash and filthy around your peers.
See also: dad spooner, granny grinder
We needed to leave but Kevin was busy being a mum bummer and we got there ten minutes late.
I'm wasn't mum bumming, she was just asking me all these questions.