Yours, Etc.: Origins and Uses of 8 Sign-Offs
/nil/ 1. New Implementation of Lisp. A language intended to be the successor of MacLisp. A large Lisp, implemented mostly in VAX assembly language. A forerunner of Common LISP.
["NIL: A Perspective", Jon L. White, MACSYMA Users' Conf Proc, 1979].
2. Network Implementation Language. Strom & Yemini, TJWRC, IBM. Implementation of complex networking protocols in a modular fashion.
["NIL: An Integrated Language and System for Distributed Programming", R. Strom et al, SIGPLAN Notices 18(6):73-82 (June 1983)].
3. Empty list or False. In Lisp, the empty list (or "nil list") is used to represent the Boolean value False. This is possible because Lisp is not typed. True is represented by the special atom "t".
4. Spoken in reply to a question, particularly one asked using the "-P" convention it means "No". Most hackers assume this derives simply from LISP, but NIL meaning "no" was well-established among radio hams decades before LISP existed. The historical connection between early hackerdom and the ham radio world was strong enough that this may have been an influence.