I'm sure I've missed something quite obvious here, but I've always been told that adonai is "my Lord", and so I always assumed that it was adon with 'my' stuck to it. But, that would be adoni, with a hireq yod pronominal suffix, like in Gen 23 where the Hittites and then Efron address Avraham. Or, if it is 'adonai' as a type 2 pronominal suffix, it would be a pathach yod, and not the qamets yod we see in the text like in Gen 15:2 where Avram addresses God. What have I missed?
chirik - adoni - "my [human] master"
patach - adonai - "my [human] masters"
komotz - adonoi - "my Lord-of-all-things" = God . Its own special grammatical construction specifically used for the name of God.