Greek, ‘Sir’, ‘master’, ‘owner’ (e.g. Matt. 25: 11), but also used in LXX to translate Hebrew Yahweh. In Christian speech it was applied to Jesus as ‘Lord’, being raised above the human level (e.g. Rom. 14: 8–9) and Paul designates the brothers of Jesus as ‘the brothers of the Lord’ (1 Cor. 9: 5; Gal. 1: 19), which seems to point to the Jesus of the earthly ministry as nevertheless a more than human being. According to Josephus the Jews refused to call the Roman emperor ‘Lord’ (kurios), because this was the title reserved for God. Paul, however, transfers to Jesus passages in the OT (e.g. Isa. 45: 23) which refer to God: it is to the Lord Jesus Christ that every knee shall now bow and whom every tongue shall confess (Phil. 2: 10 f.).
The liturgical form Kyrie eleison (‘Lord, have mercy’) became a litany in Christian worship from the 4th cent. CE.