From The Collaborative International Dictionary of English v.0.48 [gcide]: Rex \Rex\, n.; pl. Reges. [L.] A king. [1913 Webster] To play rex, to play the king; to domineer. [Obs.] [1913 Webster]
From The Free On-line Dictionary of Computing (19 Sep 2003) [foldoc]:
REX
The original name for Restructured EXtended eXecutor.