Where does “ROFLMFAO” come from?

ROFLMFAO (English) comes from English LMFAO, from English LMAO — Laughing My Ass Off; laughing my ass off.

Ancestry of “ROFLMFAO”, step by step

ROFLMFAO traces back through two separate lines of ancestry.

via English LMFAO

StepLanguageWordMeaning
1EnglishLMFAOMore emphatic form of
2EnglishLMAOLaughing My Ass Off; laughing my ass off

via English rofl

StepLanguageWordMeaning
1Englishroflused to indicate great amusement
Every word from English LMAO