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1.
Another word on the long list of words that describes an ugly girl.
Sian is a Muttbag.

She was a right muttbag, what were you thinking?
by Artbrut July 23, 2007
 
2.
A bag of weed containing multiple types
That dime I got from Barney must be a muttbag - I've felt different each time I've smoked it.
by Mgwuke M. July 05, 2011
 
3.
A plastic grocery bag full of all the loose or "mutt" beers in your fridge that you take over to a friends house instead of just picking up a case.
Joe: "do we gotta stop and get beer before we go to the party?"

Jeff: "naw we can put together a mutt bag, there are four or five keystones, three coors and a couple bud lights in the fridge."
by RJM '08 October 18, 2009