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1.
An abbreviated form for the phrase, “Laughing My Fucking Nipples Off.” Its purpose is to create a more expressive illustration of great laughter or amusement, similar to LMFAO (which means, "Laughing My Fucking Ass Off.") The term is commonly used on the Internet, online and conversations across gaming/chat rooms or texting.
Girl: Do you believe in puppy love?
Boy: I tried it once, but their assholes are too small.
Girl: LMFNO!
by samcaba January 10, 2011
 
2.
Laughing My Fucking Nuts Off
I was LMFNO when i heard that Palin is running for president.

LOLLMAOLMFAOLMNOHaHa
by georgeyea June 16, 2011
 
3.
Acronym: Laughing my fat nuts off.

May be used as a substitute for other similar acronyms such as lmao, lol, ect.
Person 1: Yeah, and then he STEPPED in it!!

Person 2: LMFNO!
by Dritto December 14, 2004