As Alfredo Edgardo said, is a Greek word, case vocative of kyrios kyriou which means Lord, master, owner of slaves and in the Christian context the Lord, Jesus, Christ. Mass is said this word with the imperative of the verb eleeo which means having compassion, have mercy: Kirie eleison, Christe, eleison: Lord, have mercy, Christ, have mercy. This invocation existed before Christianity. Kyrios nominated to any God or Emperor.
