nun (n.)

Old English nunne "woman devoted to religious life under vows of celibacy, poverty, and obedience to a superior," also "vestal, pagan priestess," from Late Latin nonna "nun, tutor," originally (along with masc. nonnus) a term of address to elderly persons, perhaps from children's speech, reminiscent of nana (compare Sanskrit nona, Persian nana "mother," Greek nanna "aunt," Serbo-Croatian nena "mother," Italian nonna, Welsh nain "grandmother;" see nanny).