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.
When someone unexpectedly engulfs their mouth over another while cleansing the other persons gums, teeth, and tongue with the other persons tongue. AKA the worst kiss someone will experience in their lifetime. May cause a break up, confusion, dislike to kissing, or someone to like the opposite sex.
Dude 1- "I was totally mouth massacred last night"
Dude 2- "Wait what?!"
Dude 1- "Yeah, her tongue was cleaning my teeth, and I couldn't even breathe because her mouth was completely over mine. It killed my mouth and liking for kissing".