[Sammet 1969, p. 506].
1. <storage, jargon> 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 microtapes were 1, 2, etc. and attempting to flap device 0 would instead start a motor banging inside a cabinet near the disk.
The term is used, by extension, for unloading any magnetic tape. See also macrotape. 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.
2. <networking> See flapping router.