Top definition
The viscous, disgusting gunk that collects on one's lip after;
A. Sleeping
B. Having lip balm on for an amount of time.

Typically, mouth butter is removed by placing the thumb and index finger on both sides of ones mouth, pinching and pulling toward the end of the preferred lip. Mouth butter often occurs on the bottom lip--rarely is it seen at the top.
"Ugh, I've got mouth butter. This shit is gross."
by SuperRank March 08, 2010
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