WTDD

WTDD What the dog doing
Refers to the popular king bach vine, where he asks his friends if they want a mint and then hillariously, a dog paw comes up and tries to take one too and that's where this saying comes up, the shortened version can be used in various texts.

King Bach : Who wants a mint?

His friends : Me, I do

They reach over to the mints, then a dog tries to take one too.

King Bach : What the dog doing?
Shortened WTDD.