1. A slang term meaning simply moustache. Named after the character from Super Mario Bros (Mario doesn't have the same ring to it).
2. "Growing a Luigi". A term for the stubble found on adoloscent boys when they attempt to grow a moustache after entering puberty.
1. Andrew: Nice Luigi you have there, Matt.
Matt: Thanks man, I trimmed it yesterday.
2. John: Tim, you growing a Luigi? At least you're trying.