meaning "what do you
mean?"
it is a shortened version on "what are you going on about?" or "what are you talking about?"
usually used when someone is not making sense for extended periods of
time or if you feel like you've missed
something.
can be lengthened to show complete and utter confusion.
example 1.
knobhead: dude. i totally just pwned that n00b. i asdfghjkl; the ask;nfw. fo shizzle my
nizzle dizzle.
awesome: what you on about?
example 2.
knobhead: so, tomorrow, then?
awesome person: what?
knobhead: we can go and you can
meet them, yeah?
awesome person: what you on about?
example 3.
Gavino Free(roosterteeth): what you on about?