n., obs. 1. On an MIT space-cadet keyboard, use of all four of the shifting keys (control, meta, hyper, and super) while typing a character key.
2. On a Stanford or MIT keyboard in raw mode, use of four shift keys while typing a fifth character, where the four shift keys are the control and meta keys on *both* sides of the keyboard. This was very difficult to do! One accepted technique was to press the left-control and left-meta keys with your left hand, the right-control and right-meta keys with your right hand, and the fifth key with your nose.
Quadruple-bucky combinations were very seldom used in practice, because when one invented a new command one usually assigned it to some character that was easier to type. If you want to imply that a program has ridiculously many commands or features, you can say something like:
"Oh, the command that makes it spin the tapes while whistling Beethoven's Fifth Symphony is quadruple-bucky-cokebottle."
See double bucky, bucky bits, cokebottle.