b> I am trying to understand the etymology of the word "Bhikkhu".
b> I would think it is derived from "Bhikkhaa"(alms food), ie one who
b> beg's for alms(?).
To be more exact, from the root 'bhikkh' (Sanskrit 'bhik.s').
b> What would be the grammatical explanation for the
b> feminine Bhikkhaa becoming the masculine Bhikkhu?
Firstly, there's no becoming since the derivation is from the root.
Secondly, I don't think there's any grammatical explanation for the
ending and gender of this word in Pali. Bhikkhu just is and was
masculine, as monastic life mostly is and was.
There is feminine form, Bhikkhunii, with a characteristically feminine
ending.
The subtleties of ethymology can be only guessed by those who dare to
research the maze of Proto-Indoeuropean language.