In Genesis 15:2, Abram uses the term Adonai when addressing the Lord. It is my understanding that this word is an emphatic of Adon, which means Father.
What do we know about the cultural and/or theological usage of the use of Adonai at that time?
The Hebrew text of Gen. 15:2 states,
The Hebrew word אָדוֹן (adon) does not mean "father," but "master," "lord." For example, Sarah said that her husband Avraham was her אָדוֹן (Gen. 18:12). Obviously, Avraham was not her father, but he was her master, for a man's wife was considered his servant and possession.
The Hebrew word which is translated into English as "father" is אָב (av). Furthermore, אָדוֹן and אָב are not morphologically related based on their root.
אֲדֹנָי is related to אָדוֹן. According to Gesenius, both are likely derived from the root ד-ו-ן,1 which when conjugated as a verb and used transitively, means "to subject to one's self, to rule, to judge."2 As mentioned before, אָדוֹן itself means "master," "lord," and it's also equivalent to the English words "sir" and "mister."
Pronominal suffixes can be suffixed to אָדוֹן to indicate possession. For example, the word אֲדֹנִי (adoni) is translated as "my lord," and אֲדֹנַי (adonai) is translated as "my lords."
Now, at first glance, you might think that אֲדֹנַי (adonai) is the word you mentioned in your original post. It's not. That word would be אֲדֹנָי. The only difference is that one has a patach ַ under the letter נ, while the other has a kamatz ָ under the letter נ. However, they are pronounced the same, and before the Masoretes introduced nikkud (vowel points) in scrolls, they would have looked identical.
So, the question is, what does אֲדֹנָי (note the kamatz) mean?
There is more discussion after that, so I recommend reading the cited reference. To note, for whatever it is worth, I agree with Gesenius in that אֲדֹנָי means "my lord" (also, אֲדֹנָי is only used in reference to Yahveh and never to men). But, it would be acceptable to translate it as "Lord," just like the French monsieur is understood to be the equivalent of the English "sir" (rather than "my sir").
1 אָדוֹן, p. 12
2 דּוּן, p. 193
3 אֲדֹנָי, p. 12
Gesenius, Wilhelm; Robinson, Edward; Tregelles, Samuel Prideaux. A Hebrew and English Lexicon of the Old Testament including the Biblical Chaldee. London: Samuel Bagster and Sons, 1857. | pdf |
Verse 15:2 says:
וַיֹּאמֶר אַבְרָם אֲדֹנָי יֱהֹוִה מַה תִּתֶּן לִי וְאָנֹכִי הוֹלֵךְ עֲרִירִי וּבֶן מֶשֶׁק בֵּיתִי הוּא דַּמֶּשֶׂק אֱלִיעֶזֶר
The phrase at issue here is "אַדֹנָי יֱהֹוִה" -- the word on the right being "Adonai" which literally means "my lords" (note plural), but in context is a reference to God. The word to the left is the Divine four-letter Name of God, or as otherwise known the "Tetragramaton." The Jewish custom for nearly 2000 years is to not pronounce the Tetragramaton, because of its Holy characteristics, but to say "Adonai."
In the singular, the word "adoni" is simply an honorific like saying, "my dear sir." For example, when Avraham goes to Hebron to find a grave for his wife, Ephron the Hittite addresses Avraham as "adoni" at Gen. 23:11 and 15. An honorific when addressing G-d, therefore, deserves something at least akin to adoni, but somehow it seems to be inappropriate to address God with a term you would also use to a stranger in business. Rabbi Sampson Raphael Hirsch, a famous 19th century rabbi and Biblical commentator, explains the use of the plural: He wrote that Avraham did not use the phrase "adoni" because he was not subservient to anyone [note that he does not address Ephron as "adoni" because he was not subservient to Ephron] but he is subservient to God, his only master, which is the same as saying that God is "all of his masters."
Avraham also uses the term "Adonai" because he is about to ask the King of Kings a question, and he wants to make it clear that he will continue to be God’s servant and obey God no matter what; he merely would appreciate some information, if it wouldn’t be too much trouble.
Let me also point out that Genesis uses several Names for God. The Creation story uses two in particular -- the Tetragramaton (יהוה) and the word "Elohim" (אלוהים). In English Bibles you'll notice that the phrase"The Lord" is used to translate the Tetragramaton, and the word "God" is used to translate "Elohim." Jewish tradition explains the different terms as describing two distinct attributes of the Creator. When God is acting with His attribute of Justice, the Torah uses the word "Elohim." But when God is acting with Mercy, then the Tetragramaton is used. During the first six days of Creation, the Torah tells us that Elohim is the Creator. Then, at Gen. 2:4, there is a transition where the Torah explains that יְהוה אלוהים -- invoking both Names -- made earth and heaven. Thereafter, the Torah tells of the Creation of Man using the Tetragramaton to describe the Creator. Commentators have noted the importance of this noting that had we been created by God's attribute of Justice, instead of his attribute of Mercy, we could not have survived long in the world.
Back to Avraham -- it is important to note that throughout his story, Avraham/Avram refers to God by either the Tetragramaton, Adonai, "Eil Elyon" ("God Most High" see Gen. 14:22), and "Eil Shaddai" ("God Almighty" Gen. 17:1), and completely avoids the term Elohim.