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Latin word rex comes from Proto-Indo-European *rek-, and later Proto-Indo-European *h₃rḗǵs (King, ruler.).

Rex etymology history?

Rex detailed word origin explanation

Dictionary entryLanguageDefinition
*rek- Proto-Indo-European (ine-pro)
*h₃rḗǵs Proto-Indo-European (ine-pro) King, ruler.
*rēks Proto-Italic (itc-pro) King.
rex Latin (lat) (Late Latin, chess) king. King, ruler.