This is an expression you see every once in awhile intended to mean "any UNIX-like system." The name arises from a somewhat lame technical joke involving wildcard expansions; as interpreted by most of the command-line shell programs used on such systems, the expression "*n?x" would refer to "any file whose name ends with an N followed by another character followed by an X", which is a pattern that seems to be shared by the names of almost all of the unix-like operating systems. For example, the pattern applies to UNIX, but also to linux, IRIX, minux, SIOUX, the POSIX standard, etc. It would technically apply to BSD, because BSD is considered to be equivalent to UNIX, but not to multics because multics is not a unix and does not even contain either the letter N or the letter X.

The term *n?x was originally developed as a safeguard; sometimes on an BBS or IRC, using the term "UNIX" to mean "UNIX-like system" will result in being flamed by pedantic people who want to point out that linux is technically not a UNIX. Thus, the term "*n?x" was thought up in order to be used as a replacement for "unix-like system" that didn't take as long to type. These days you mostly see the term used on technical forums such as slashdot or everything where people either use it offhandedly as a toss-off joke, or else snidely use the term for the express purpose of making themselves appear intelligent and technically proficient, as if to say, "Look! I know how to use a wildcard expansion! I'm smarter than you!". I use the term quite a bit, but i'm not sure which reason applies in my case.

(A few sample regexp equivilents of *n?x, by the way, would be /.+n.x/. Or /\w{1,4}n(?:i|u)x/. Or /.+n\wx/. But you won't be using any of them, so it doesn't matter.)