Laughing. At. A. Fat. Kid.
Can be used in the litteral term , where you are actually laughing at an obese person (no disrespect to the rounder ones out there) or an other word to use on online conversations to replace the common LOL, ROFL or LMAO.
1) I saw this girl walking down the street and i had to stop to LAAFK.
2) That was so damn funny, i'm LAAFK-ing.