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1.
Laughing My Mustache Off. Used online or in person.

When extreme hilarity causes the facial hairs of the philtrum to jump off.
Mario: Mama mia, I have fallen into a large green pipe!
Luigi: LMMO!

And thus, Luigi and Mario had one less thing in common.
by dreakaye November 06, 2011
 
2.
laughing my mittens off
Nargis: dude are you wearin mittens?
Ramjad: yeh and you just said somethin funny so I'm lmmo!
by AlllllP March 24, 2007
 
3.
1. Laughing my maps off.
In a moment of extreme hilarity, a map bearing person might find themself laughing so hard that their maps come off.
Mary: Hey, my boyfriend's name is Ignatius!
Anne: Lmmo!!
by Scotlyn Smuggletea August 17, 2009