'flak meant a swift & incessant barrage from any sort of gun — a nearer synonym, tho not exact, would be "crossfire"'. I don't agree. In English flak is anti-aircraft fire, crossfire is crossfire, bombardment is bombardment. Flak has come to mean criticism metaphorically "I'll get some flak for that!".

Nine-ten might it be from 'nine-ten and out' - the way that boxing referees count out boxers. Still don't know what it actually means though.