BEagle's correct. In the good old days, briefing room chairs were wooden fold-ups - a bit like a deckchair. At the end of a session we folded the chairs up and stacked them at the back of the room on our way out. 'Stacking' was a general expression used by all ranks, in all trades from Aircrew to RAFP. As slang goes I think that, along with 'Gen' or the more specific 'Pukka Gen', its as old (?)as the RAF itself.