WordNet (r) 3.0 (2006):

POB
    n 1: a numbered compartment in a post office where mail is put
         to be called for [syn: Post-Office box, PO Box, POB,
         call box, letter box]

The Free On-line Dictionary of Computing (30 December 2018):

Prisoner of Bill
PoB

    (PoB) A derisory term, in use generally among Unix
   users, for anyone who uses Microsoft products either because
   they don't know there is anything better (i.e. Unix) or
   because they would be incapable of working anything more
   complex (i.e. Unix).

   The interesting and widespread presumption among users of the
   term is that (at least at the time of writing, 1998) using
   anything other than Unix or a Microsoft OS (whether VMS,
   Macintosh, Amiga) is so eccentric a choice as to be at
   least somewhat praiseworthy.

   (1998-09-07)