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2.
Laugh my fucking dick off.
Used in really any case where your friend or enemy or pet turtle texts you something funny.

Should not be used in cases where the person will blackmail you for laughing at what they said. Shown in example #2.
1)While texting:

Person 1: "Damn she looks mad ratchet"
Person 2: "Lmfdo! Dude you're hilarious, that's so true though..."

2)

Person 1: "She looks like she gained some weight, poor girl. I heard she caught herpes too, shortly after her mother was killed in that car accident yesterday."
Person 2: "Lmfdo! Sucks to suck."
Person 1: "You're an asshole! We're no longer friends! I'm telling the whole school about how you're an ass-licking mother fucking gay rhino-raping fat piece of vanilla flavored shit in a dumpster full of fat rats drinking kool-aid riding unicycles while singing random justin bieber songs.... No nipple having bitch!!!"
by Little Bit of Luck January 08, 2013
 
1.
LMFDO

Online term short for Laughing My Fucking Dick Off or possibly Laughing My Fat Dick Off. Term used rarely online. Most likely derived from LMFDO, LMFDO was probably used to differentiate the person saying it from regular online users who simply use the occasional LOL or LMFDO
When little Broderick fell of the swing into a pile of mud I was LMFDO!
by Sickopuppie June 05, 2007
 
3.
LMFDO

Online term short for Laughing My Fucking Dick Off or possibly Laughing My Fat Dick Off. Term used rarely online. Most likely derived from LMFDO, LMFDO was probably used to differentiate the person saying it from regular online users who simply use the occasional LOL or LMFDO
When little Broderick fell of the swing into a pile of mud I was LMFDO!
by Sickopuppie June 05, 2007
 
4.
laughing my fucking dick off. this can be used instead of lol. but it is something that a boy would use.
her head was huge.lmfdo.
by daeqwan October 15, 2008