rex (n.) Look up rex at Dictionary.com
"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).