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3.
Like a beer belly, beer butt is the result of consistently drinking large quantities of beer. Beer butts are large and often saggy and/or jiggly.
Mer: Have you seen the beer belly on Ben?

Cassie: Ha ha, yeah, but that's nothing compared to his beer butt!

Mer: Oh yeah, that thing is HUGE.
by Mer B March 28, 2008
 
1.
Diarrhea that normally follows a long night of drinking cheap beer.
I wish I didn't drink all that Pabst last night, the beer butt is killing me.
by Beef Wellington September 22, 2007
 
2.
The last swig of a beer left in a bottle or glass, it's butt.
Almost all of the beer had been drunk, what remained was the beer butt.
by n0misain June 03, 2009