vt. 1. 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,... 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.
See also macrotape.
Modern cartridge tapes no longer actually flap, but the usage has remained.