Is there a feminine form of the word bastard? It seems like bastard is a word that’s applied to male children only.
Talking about the archaic use of bastard to mean “illegitimate child”, it is not specifically masculine. One can talk about a “bastard daugther” just as much as one can talk of a “bastard son” or “bastard child”, as Google ngram shows. Well, not as much, because lineage or legitimacy is not nearly as important for daughters as it is for sons, but still, such usage is attested.
The human rights organization Bastard Nation suggests the term has always been gender neutral. Certainly a bastard child could be male or female.
Typical with religious institutions, Latin terms were used in Parish birth records:
Nothus (gnothus) bastard, illegitimate
Filia nothus for female girl
Filius nothus for male child