"Laughing my boobs off"
"Laughing my balls off"
They're all implying the same thing; you're laughing incredibly hard. It's often a replacement for "lmao" (laughing my ass off) for people who prefer not to curse in their interweb acronyms.
Person B: LMBO
Person A: wait, but you're a girl, you don't have balls!
Person B: it can also mean "laughing your boobs off"
Person A: what if you have neither? (somehow)
Person B: then it can mean, "laughing your butt off"