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 called "Buckies" after Niklaus Wirth, a computer scientist whose nickname was "Bucky." Wirth coined the term at Stanford University in 1964 after suggesting to use an additional key to set the eighth bit of a 7-bit ASCII (American Standard Code for Information Interchange) 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, known as the "bucky bit." Therefore, pressing two keys at once is called entering a double bucky. For instance, "doubly bucky F" might refer to pressing Ctrl+Alt+F.