What does lmfao mean?

The acronym "lmfao" is a more profane variant of the acronym "lmao", which means "laughing my ass off". The former adds the "f" representing the adverb form of the classic four-letter profanity.

While "lmfao" is more profane than "lmao" it is also intended to be more exclamatory. In other words, when something is moderately funny, one would use "lmao", but if something were incredibly funny, "lmfao" would be more appropriate. The easiest way to understand how to use the two variants is to see them used in the same message in succession. Here is an example:

Messager 1: "So we're sitting in class and Marta is in front of me and she farts! Ms. Reynolds looked like she was going to kill her."

Messager 2: "Lmao!"

Messager 1: "And like two seconds later, Marta gets up and asks to go to the bathroom and Ms. Reynolds lets her and on her way out, Marta pulls her shorts out of her crack and she's got this nasty stain!"

Messager 2: "Lmfao!"

As you can see, the basic concept is that if there are two things in a sequence and one is funny and the next one is funnier, "lmfao" elevates the level of exclamation. Here's one more example:

Messager 1: "Jerry lost $100 at poker. Again."

Messager 2: "Lmao."

Messager 1: "He lost most of it calling an all-in with a bluff."

Messager 2: "Lmfao!"

If you're at all familiar with poker, calling an all-in when you are bluffing is among the stupidest things you can do, so the second comment in this exchange requires an additional level of exclamation.

Although the distinction between the two acronyms is obvious, that distinction is often lost on those who use it as Internet culture has shifted the acceptance of profane usage to the point where it is readily accepted. Therefore, those who would be better to choose the lesser of the two variants in terms of their exclamatory power choose the more profane variant because that is what is normal for them. Thus, the normal is now the profane.