The Collaborative International Dictionary of English v.0.48:

Rex \Rex\, n.; pl. Reges. [L.]
   A king.
   [1913 Webster]

   To play rex, to play the king; to domineer. [Obs.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

Rex
    n 1: a male sovereign; ruler of a kingdom [syn: king, male
         monarch, Rex] [ant: female monarch, queen, queen
         regnant]

The Free On-line Dictionary of Computing (19 January 2023):

REX

   The original name for Restructured EXtended eXecutor.