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The flattening and/or widening of the ass after child birth...
"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...)
by 420 PPL August 23, 2008
When a young girl has an ass like a 60 year old mother of 6.
I would've hit that, but her mum bum was mingin'
by Badmon22 September 16, 2011
someones mum who is a legend.
"mumbums here"
by chloemayy June 08, 2009
a girl wit no ass, a saggy ass, or jus an ass that looks something like a 50 year old womans.
Steph shouldnt wear tight jeans because she has a mum bum and its discusting
by Sh@w_t@y June 28, 2006