1. exclamation of confusion or lack of understanding
2. statement requesting the other conversant to repeat him/herself or better explain said statement, or elaborate on the thought process
joe: DUDE what is that STUFF in your hair?!?
sean: what? what stuff? i don't have stuff in my hair
joe: uhh...yeah you DO. you've totally got pediculosis capitis, man. that's NASTY.
joe: dude you've got LICE.