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1 definition by 420 PPL

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