Yours, Etc.: Origins and Uses of 8 Sign-Offs
"a king," 1610s, from Latin rex (genitive regis) "a king," related to regere "to keep straight, guide, lead, rule," from PIE root *reg- "to rule, to lead straight, to put right" (cf. Sanskrit raj- "king;" Old Irish ri "king," genitive rig; see regal).
The original name for Restructured EXtended eXecutor.