Something that is so freaking fly and fresh. If someone's singing sounds like Jesus and Fergy mixed together, their voice is Guac. When you want to describe something as the most amazing, on fleek, dope ass, gucci thing ever just use the word instead of trying to explain how flipping legendary the said thing is. In other words Guac is the new "dope" for you older users.
Person one: "Damn! That is the most delicious, gucci, awesome, beautiful, amazing, exhilarating, dope ass, on fleek steak I have ever tasted! It was a legendary slab of heaven! I had an out of body extra terrestrial experience just from eating it! Wow!"
Person 2: "So you mean it was Guac?"
Person 1: "Guac af man. Guac af."