From WordNet (r) 3.0 (2006) [wn]:
Rex
n 1: a male sovereign; ruler of a kingdom [syn: king, {male
monarch}, Rex] [ant: female monarch, queen, {queen
regnant}]
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 (8 July 2008) [foldoc]: REX The original name for Restructured EXtended eXecutor.