>we know about
> "dominicus" meaning "of the lord" but I don't know any "domenica" with
> the meaning "of a feminine lord".
It doesn't. It means feminine thing of a lord.
dominicam: occurs in Afran ap Isid Differ. 86
dominicae: occurs in Varro R. R. 2:10:10
Dominica Dies = Sunday occurs in the Vulgate and in Tertullian
dominica cena = last supper occurs in the Vulgate.
The change of short i to an e is absolutely regular and it woudl be
something to remark on if it did not happen. There is no problem with
domenica = feminine thing of a lord.
As for -us based on Greek -os, thank you for the chuckle. Made my day.
Peter