Although not the most common abbreviation on the Internet, IANAL is sometimes used on sites such as Twitter, Quora, and Reddit. But what does IANAL mean, and why do people say it?
I am not a lawyer
IANAL is an internet initialism meaning «I’m not a lawyer». It is commonly used by non-lawyers who want to clarify that their legal opinions are not legal advice. Clarifications like this are important online, as you may face lawsuits or prosecution for providing legal advice (or acting as a lawyer) without a legal license. (However, non-lawyers rarely face real consequences for casual legal advice.)
In some cases, IANAL is used by incumbent lawyers who wish to issue free legal opinions without forming an attorney-client relationship. Entering into an online attorney-client relationship is not just an inconvenience, but can also be illegal, as a lawyer may not have jurisdiction to practice law outside of their state.
YANIL and TINLA
«IANYL» is similar. It means «I’m not your lawyer». This can be used by someone who wants to clarify that they are a lawyer but do not provide professional legal advice or are in an attorney-client relationship with you.
Similarly, «TINLA» means «This is not legal advice.» People use these expressions to clarify that they do not provide professional legal advice online.
Do people actually say IANAL?

IANAL is an incredibly unpopular piece of internet slang. Most people have never heard of it, and it doesn’t make much sense outside of Quora or the Reddit /r/legaladvice forum. So why IANAL?
Oddly enough, IANAL has been around since the late 80s. It was a semi-popular phrase about Usenet and ARPANET, the two predecessors of the Internet. As history suggests, pre-Internet lawyers coined IANAL to clarify that while they were giving legal opinions online, they were not entering into an attorney-client relationship or acting outside of their jurisdiction. (It could also be an accidental reference to the proto-meme «I’m not a doctor, but I play on TV».)