rex (n.)

"a king," 1610s, from Latin rex (genitive regis) "a king," related to regere "to keep straight, guide, lead, rule" (from PIE root *reg- "move in a straight line," with derivatives meaning "to direct in a straight line," thus "to lead, rule;" source also of Sanskrit raj- "king;" Old Irish ri "king," genitive rig).

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Definitions of rex from WordNet

Rex (n.)
a male sovereign; ruler of a kingdom;
Synonyms: king / male monarch