Top Definition
one person getting railed/dry-humped/oral sex by a single unit that happens to have a hairy upper lip.
guy #1: what did you do last night?"
guy #2: i mustached monorailed some bitch all night long!
guy #3: trust me, you're welcome. do not mustache monorail my mother ever again!
guy #2: deal.....
by eatshitanddie666 May 30, 2010

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