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.
by barkbat October 22, 2009

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