1. [obs.] The bits produced by the CONTROL and META shift keys on a SAIL keyboard (octal 200 and 400 respectively), resulting in a 9-bit keyboard character set. The MIT AI TV (Knight) keyboards extended this with TOP and separate left and right CONTROL and META keys, resulting in a 12-bit character set; later, LISP Machines added such keys as SUPER, HYPER, and GREEK (see space-cadet keyboard).
2. By extension, bits associated with ‘extra’ shift keys on any keyboard, e.g., the ALT on an IBM PC or command and option keys on a Macintosh.
It has long been rumored that bucky bits were named for Buckminster Fuller during a period when he was consulting at Stanford. Actually, bucky bits were invented by Niklaus Wirth when he was at Stanford in 1964--65; he first suggested the idea of an EDIT key to set the 8th bit of an otherwise 7-bit ASCII character). It seems that, unknown to Wirth, certain Stanford hackers had privately nicknamed him ‘Bucky’ after a prominent portion of his dental anatomy, and this nickname transferred to the bit. Bucky-bit commands were used in a number of editors written at Stanford, including most notably TV-EDIT and NLS.
The term spread to MIT and CMU early and is now in general use. Ironically, Wirth himself remained unaware of its derivation for nearly 30 years, until GLS dug up this history in early 1993! See double bucky, quadruple bucky.