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.