Today's Advent antiphon begins by addressing the Christ who comes by the Hebrew title which means Lord / Master, used in prayer to the Almighty, to acknowledge divine power and sovereignty.
The word first appears in the story of Moses and the burning bush. 'Adoni', the root of the word, originates in the Phoenician term used to address their deity Tammuz as Lord. It was also re-used by the Greeks in the name of the deity Adonis, beloved of Venus, and also appears in other ancient Eastern Mediterranean religious cults. Sea-faring Phoenicians were great exporters of their culture. One intriguing historical reference places the cave Jesus was said to be born in (where the Bethlehem church of the nativity was built) on the site of an ancient sanctuary to Adonis-Tammuz.
Religious belief systems in those times didn't compete with each other, unless they were associated with a struggle for power and dominance. They borrowed from each other's language, stories, sacred places, rituals and symbols to express perpetually evolving relationships with the divine and each other. Syncretism is often used as a term of disrepute in relation to borrowed or imitated religious practices. In reality it's what all human beings do, except when insecure enough to try and prove they are exceptional and 'pure' through zeal and devotion in a bid for dominance.
Moses fled from retribution for a killing into the Sinai desert and was surviving there as a lonely shepherd. Here he received the call to return and liberate the children of Israel. Answering this call rescued him and transformed him into the leader, law-giver and trusted friend of God he became. The coming Christ is identified with God's self manifestation to Moses in the mystery of a bush which burns and is not consumed - a symbol of Christ's incarnation - it points to the fire of divinity inhabiting the humanity of Jesus without consuming Him.
'Adonai' whatever its religious origins expresses the reverence and loving devotion proper to relationship with God. It is an appeal to the Almighty to deal with us in our great need with the infinitely greater power of devoted love that belongs to the Author of our being. It expresses that ultimate trust which gives life its true vitality.