The response to a question that you know the answer to but don't want to admit, the response to a question you don't know the answer to or just a response to piss someone off.
"Can you help me reformat my hard drive?", ummm...
usualy said by a bloke when asked a question. any question as a matter of fact.
it can also mean the following:
err no way!
Girl: Do you have a crush on me?