1. Acronym for Jack Of All Trades. A person who is somewhat proficient at a number of different things. However the phrase is usually used in conjunction with "but master of none" putting a negative connotation on an otherwise positive phrase.
2. A collection of live, unreleased, and rare versions of music by recording artist Jack Johnson.
My uncle is a JOAT. He fixed our plumbing and then worked on his hot rod for a while.