Nanna 1

Nanna is also Estonian and Finnish. The name day for Nanna in Estonia is August 15. The name day for Nanna in Finland July 9.

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Sources: https://www.stat.ee/public/apps/nimed/
https://fi.m.wikipedia.org/wiki/Nanna_(nimi)
https://almanakka.helsinki.fi/en/name-days/name-day-search.html
https://forebears.io/forenames/nanna (I think)

Also Finnish name. Finnish name day July 9.
https://fi.wikipedia.org/wiki/Nanna_(nimi)
https://almanakka.helsinki.fi/en/name-days/name-day-search.html

Also Estonian (albeit archaic): https://www.stat.ee/public/apps/nimed/

Nanna is only slang for grandmother in English-speaking countries. In Sweden and Norway grandmother is called "Mormor/Farmor" and the most common slang is Mommo or Fammo. It has nothing to do with grandmothers where it's used.