4.2

4.2 n.  Without a prefix, this almost invariably refers to
   {BSD} UNIX release 4.2.  Note that it is an indication of
   cluelessness to say "version 4.2", and "release 4.2" is rare;
   the number stands on its own, or is used in the more explicit forms
   4.2BSD or (less commonly) BSD 4.2.  Similar remarks apply to
   "4.3" and to earlier, less-widespread releases 4.1 and 2.9.,
   and will doubtless apply to 4.4 in the near future.