I was watching a movie with my wife, whose first (and main) language is French. On character asked another something along the lines of "Can I ask you a question?". She replied "Shoot."
That was rather confusing for my wife, who thought that maybe it implied some sense of speed/urgency. When I explained it really is just idiomatic for "Yes" or "Go ahead and ask," I had a very hard time explaining why we would use the word "shoot" for that. And an even harder to explaining the subtlety of it versus "yes" or "sure" or "go ahead."
Anyway, what is the origin of using the word "shoot" to mean it is okay to ask a question? And as a sub-question, is it used in all English, or is it particular to American English and others use different idiomatic expressions for the same thing?