“Whar doon” is a shortcut for the phrase, “What are you doing” and acts as its slang replacement.

It’s mostly used in the form of a question, but doesn’t have to be.
Me: Hey, whar doon?
My sister: Drugs hbu?

Me: Whar doon tonight? Wanna catch a movie?
My sister: I’m bust tonight! Whar doon tomorrow?
Me: I don’t care whar doon tomorrow because I wanted to hang tonight.
by AngieCWmusic September 25, 2017