Q. Why is a rabbi’s wife called a rebbetzin?
A. I don’t think anyone knows for certain. Strictly speaking, it is only the rabbi himself who should have a title. It is he who has gained the rabbinical qualifications, not his wife, though most rabbis’ wives are also very knowledgeable. Nonetheless, the rabbi and his wife almost always form a rabbinical partnership team in which both are involved in teaching and leading the community. Hence it is appropriate for her to have a title that recognises her status.
The term rebbetzin obviously comes from Yiddish (as does chazan-te for a chazan’s wife and shammes-te for a shammes’s wife). In Hebrew it is usual to call her rabbanit, though this term may originally have meant a rabbi’s daughter or a woman who was a rabbinic scholar in her own right. The fact that rabbis often marry the daughters of other rabbis may have led to all rabbis’ wives being called rabbanit.