Verbal response to a bone question asked by a superior. Bone questions asked by juniors are dealt with differently.
Further to earlier explanations, within the British Army, also used in response to an answer to an obvious question. This is a question that you have set the other person up to answer. Can become quite frustrating as you become paranoid answering any question in case it is a Wah!
Steve: "Is that a can of beer in your hand Bill?"
Bill: "Bugger, been Wah'd again!"
The fact MO's are very economical in their labours would tell you a story.