It usually stands for "as fuck" and is used to intensify the meaning of the word or phrase that comes before it. For example: It's hot af (as fuck) outside! (It's extremely hot outside) I am tired af (as fuck) right now! (I'm so tired right now.) Note that this kind of phrasing is very rude and is mostly used among friends or in a group that wont mind the use of such language. You'll most often see it abbreviated as "af" when written. This kind of slang has existed for a long time, but it has only recently been abbreviated when written on the internet.

It means "as fuck" -- which is used to emphasize whatever you're talking about. For example, if someone says "he's funny AF" that means he's extremely funny. [emoji]