A double bucky is a term for pressing two modifier keys on a computer's keyboard at the same time. Historically, the modifier keys (Ctrl and Alt, for instance) are referred to "Buckies" after Niklaus Wirth, a computer scientist whose nickname was "Bucky." The term was coined at Stanford University in 1964, after Wirth first made the suggestion that an additional key be used to set the eighth bit of a 7-bit ASCII character when typing. The addition allowed for a greater variety of input characters without an unreasonable number of keys on the keyboard.
Pressing a modifier key changes the eighth bit, which is known as the "bucky bit." Therefore, pressing two keys at once is referred to as entering a double bucky. For instance, "doubly bucky F" might refer to pressing Ctrl-Alt-F.