TL;DR: AITA is an acronym for the phrase, “Am I The Asshole?” popularized on the eponymous subreddit, where people ask if they were the ahole in a particular situation.
Am I The Asshole?
The rule in the subreddit is that every post begins with AITA. This is because if the post were to surface in r/all (meaning it was very popular), the “AITA” would help distinguish it from all of the other posts from all of the other subreddits. Plus, it just makes reading the normal subreddit take a bit less mental work.
AITA is also used as a hashtag on twitter, though not very frequently. Dril, a large twitter account, recently tweeted it, so it’s certainly possible it will take on wide use, beyond just reddit.
AITA also has its own UrbanDictionary definition, which defines it about as we do, adding, “Also, it can be used in a sarcastic tone to indicate that it is truly someone else who is being mean or unreasonable.” This sarcastic or ironic use is something like this:
It is a somewhat common critique of the subreddit that there are not enough aholes in it. Many people see a post that is written by an asshole, where that person is an asshole, and finding that to be a negative thing, instinctively downvote it. This, however, is explicitly against the rules of the subreddit, since one of its stated goals is to expose assholes and asshole behavior.
Do not downvote the post even if he is an asshole! We want assholes to be visible, not swept under the rug. Go to the comments section and call him an asshole instead. - AmITheAsshole, Subreddit rules
Further, one Redditor conducted a study and found that most comments (on threads with 20+ comments) were replying “no” to the question of AITA.
It seems /r/AmITheAsshole conforms to Betteridge’s Law of Headlines—any article whose headline is a question can be answered with the word ‘no’—as in most posts the OP is almost certainly not the asshole, evidenced by the mode at 80-90% of replies arguing OP is not an asshole. - Source
Hopefully this has answered any and all of your questions related to the phrase, AITA. Feel free to submit a comment into the form on the Am I The Asshole Podcast homepage if you have any further questions.