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15.
Added to the end of a sentence, when in a conversation with a mate and in unusual company, to secretly direct your mate's attention to approaching person(s).

LUIGI = Look Up Instantly - Girls/Guys Incoming.
mate1: did you see those two guys with the sack?
mate2: they look very suspicious, luigi
by noddy330 June 26, 2009
 
16.
Luigi is the younger but taller brother of Mario. In Paper Mario you can read his diary where he mentions that he is terrified of boo's and that he wants to go on his own adventure. In the sequle Paper Mario and the thousand year door his wish to have his own adventure comes true. Luigi recived a letter that says Princess Eclair of the Waffle Kingdom was Kidnapped by the evil chestnut king and that they needed his help. He has to find the seven pieces of the Marvuilse compas to find Princess Eclair. In the game you can even buy a series of books based on his adventure. When Mario talks to Luigi he tells him about how he likes Princess Eclair so Daisy is going to have to find another man.
"I hope they come out with a Paper Luigi."
"Yeah that would be fun."
by TheBlackMew September 06, 2006
 
17.
The brother Mario. People seem to think he's underrated, although he has a lot of fans.
Luigi really isn't that underrated.
by MR. DOODID October 26, 2007
 
18.
The green wonder who is better than mario, and the only reason he hasn't overshadowed mario is because nintendo won't give him many chances. Luigi's mansion is one of only two in which luigi is the star. In the later games, he has higher jumping yet less traction than mario.
The man in green, LUIGI! No pipe is too out of reach, No foe is too high to clobber!
by catagon December 03, 2008
 
19.
Your right-hand bro who's always got your back. Most people think that he's little more than some loser that just tags along with you, but you know that he's there to help you out.

Named after the younger brother of the famous Super Mario Brothers.
Person 1 "Dude, who invited the fricken' geek?"

Person 2 "Back off, man; he's my Luigi."
by sorrio January 22, 2011
 
20.
Luigi is the kind of guy who would message you in the middle of the night just to tell you a crazy story that happened to him on that day or just to say that he misses you. He knows the right words to say when you're feeling down and he knows when to shut up when you don't feel like talking. He's the kind of guy who would call you when he's drunk and tell you how an amazing friend you are and how he's lucky to have you. He doesn't care if you eat like a pig or if your hair is all messed up. He doesn't mind if you can't sing. Because he loves you for who you are and not for who you're not. He would stay up late for you when he knows something's wrong and will only sleep if he knows you're okay. He would sit down and play the piano if he doesn't feel like talking or just laugh at everything that's getting out of hand. He's one of a kind and if you have a Luigi in your life, you are truly lucky.
Luigi, my bestfriend and my ninja ♥
by Ninja23 December 18, 2010
 
21.
Mario's younger brother, lives in the Muchroom Kingdom. Starred in his own game, Luigi's Mansion.
by u.ber. August 21, 2003