It is an abbreviation that refers to itself in the expression for which it stands.
In computing, an early tradition in the hacker community (especially at MIT) was to choose acronyms and abbreviations that referred humorously to themselves or to other abbreviations.
Perhaps the earliest example in this context, from about 1977 or 1978, is TINT ("TINT Is Not TECO"), an editor for MagicSix. This inspired the two MIT Lisp Machine editors called EINE ("EINE Is Not Emacs") and ZWEI ("ZWEI Was EINE Initially"). These were followed by Richard Stallman's GNU (GNU's not UNIX).
Many others also include negatives, such as denials that the thing defined is or resembles something (that the thing defined is, in fact, usually similar to or even derived from).
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