The word נדן meaning a sheath for a sword, appears once or twice in TaNa”KH. It appears unambiguously in I Chronicles 21:27:

The other occurrence is in Aramaic in Daniel 7:15, where the meaning and relationship to the Hebrew word are more murky.

KJV translates it: “I Daniel was grieved in my spirit in the midst of my body, and the visions of my head troubled me.” NJPS has “As for me, Daniel, my spirit was disturbed within me and the vision of my mind alarmed me.” so it doesn’t seem at all clear that the Aramaic word has the same meaning as the Hebrew word.  Rashi connects them: בגו נדנה. בתוך תיקה נרתק שלו כלומר נבהלה רוחי בתוך הגוף מן החלום הזה:

BDB and HALOT say that it is a Persian loan-word.  HALOT connect it to the Sanskrit nidḫāna – container.  

A similar word occurs in Ezekiel 16:33

לְכׇל־זֹנ֖וֹת יִתְּנוּ־נֵ֑דֶה וְאַ֨תְּ נָתַ֤תְּ אֶת־נְדָנַ֨יִךְ֙ לְכׇל־מְאַֽהֲבַ֔יִךְ וַתִּשְׁחֳדִ֣י אוֹתָ֗ם לָב֥וֹא אֵלַ֛יִךְ מִסָּבִ֖יב בְּתַזְנוּתָֽיִךְ׃

They give gifts to all whores: but thou givest thy gifts to all thy lovers, and hirest them, that they may come unto thee on every side for thy whoredom.

Here the word refers to a dowry (the same word נדוניא in Aramaic is used in the ketubah) or the price of prostitute, equivalent to the Hebrew אתנן.  Tawil derives the word from the Akkadian nudunnû which means dowry or gift. In Hebrew also, אתנן seems to be related to נתן, to  give.  Tawil further refers to Richard White who suggests that using the word נדוניך is a deliberate double entendre where the gift to the harlot “נדן” refers also to the vagina, the sheath נדן.  He points out that this usage is found in the Genesis Apocryphon.

There is a similar parallel with these words in other languages.  נרתיק, vagina in Aramaic and Mishnaic Hebrew is from the Greek narthex νάρθηξ which is literally a giant fennel which is a hollow tube.  Latin vagina means a sheath for a sword.