HODL is defined as a misspelling of “hold”. It has evolved into a shortened form of “Hold On for Dear Life”.

After buying crypto, a person who is HODLing intends to keep it even as prices go up and down. Originally a misspelling of “hold”, HODL became a popular term among those who buy cryptocurrencies.

A person who does this is known as a “HODLer” or “HODLER”.

You can find the original post that created “hodl” here, https://bitcointalk.org/index.php?topic=375643.0