According to www.dictionary.com Adonai is defined as: A Hebrew name for God, usually translated in the Old Testament by the word ``Lord". Note: The later Jews used its vowel points to fill out the tetragrammaton (This is the description for the four (tetra) Hebrew letters (grammata) used in the Hebrew Bible for the name of the God of Israel, usually spelled YHWH, or JHVH.) "The indescribable name," and during reading it was substituted by the word "Adonai".

Significant to the understanding of the word Adonai is the suffix `ay'. It was frequently suggested that the ending is a first person possessive suffix on a plural form of `adon' (my lord). This is logical for the form adonay, but the finely familiarized form adonay, which also comes into view in the Massoretic book, is much more difficult to explain" (Van Gemeren). There had been much consideration given...