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1) noun - a pathological liar, bullshitter, etc.

A mulch mouth will ramble on about exaggerated or untrue accomplishments as long as someone will let them.

2) Someone with horrible teeth.
1) "Hey, that guy over there says he's a top-ranking Navy Seal! Before that, he owned a hotel in Vegas, toured with Bon Jovi, wrestled an alligator, and almost died while saving a kid who was almost hit by a train!"
"Don't believe him. He's a mulch mouth."

2) She needs some serious dental work done to that mulch mouth of hers.
by Argh Nargh August 16, 2011
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