Nun
Old English nonne, from ecclesiastical Latin nonna, feminine of nonnus ‘monk’, reinforced by Old French nonne .
wiktionary
From Middle English nonne, nunne, from Old English nunne(“nun, priestess”), from Late Latin nonna(“nun, tutor”), originally (along with masculine form nonnus(“man”)) a term of address for elderly persons, perhaps from children's speech, reminiscent of nana, like papa etc. Doublet of nonna.
Borrowed from the letter’s name in the respective language.
nun
etymonline
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).
