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Used to insult a person. Refers to someone who would try to consume magic mushrooms through their anus, usually because they are retarded.
Rennie : wtf is a bum shroom?!

joanne says: i dont know : (

Rennie says: ud have to be on shrooms to think that was a real thing

joanne says: but i like it actually ! i just invented a legendary insult "here you ya fuckin bum shroom!"
by tripper1988 March 30, 2010
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