1. [obs.] To unload a DECtape (so it goes flap, flap, flap...). Old-time hackers at MIT tell of the days when the disk was device 0 and DEC microtapes were 1, 2,... and attempting to flap device 0 would instead start a motor banging inside a cabinet near the disk.
2. By extension, to unload any magnetic tape. Modern cartridge tapes no longer actually flap, but the usage has remained. (The term could well be re-applied to DEC's TK50 cartridge tape drive, a spectacularly misengineered contraption which makes a loud flapping sound, almost like an old reel-type lawnmower, in one of its many tape-eating failure modes.)