The term YMFY originates from 2 friends that have experienced new levels of high from chronic. It is an Acronym for "Yes, MotherFucker, yes!" and is pronounced letter by letter, Y-M-F-Y. This is only found in online gaming culture slang and is close to 1337 speak but much more high.
Me: mmm tasty nug man
Him: YMFY!(Yes motherfucker yes).
It's an expression of enjoyment that can be used to describe things outside the drug world.