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If I researched correctly (and with that I mean reading a bit of Wikipedia), the plant name ワスレナグサ, or 忘れな草 when written with Kanji, is a calque of German "Vergissmeinnicht" ("forget-me-not" in English) from the early 1900s. I know that グサ is just 草 having gone through Rendaku, but what kind of grammar is behind the rest of the construction? I wondered because 忘れな reminded me of a positive imperative rather than a negative one.
If anyone has an answer, I would greatly appreciate hearing it!
I researched for a while and here's my result.
According to this site:
「忘れな草」 という名称は，英語 「forget-me-not」 の和訳語ですが，植物学者の川上滝弥氏が「勿忘草」の漢字を当てて命名したものです。
英語の 「forget-me-not」 は 〈私を忘れないで！〉という意味ですが，このことばは，もともとはドイツの伝説からきていることばなのです。
「忘れな草」 is a 和訳(わやく) of the English word 「forget-me-not」, and 「勿忘草」 is a 当て字(ateji), meaning 〈私を忘れないで！〉(Don't forget me!)
However, according to this:
植物学者の牧野富太郎は「わするなぐさ（忘るな草）」と呼ぶ方が良いと命名したが、現在は「忘れな草」の別名として呼ばれる程度となっている。
「忘れな草」(ワスレナグサ) is a 別名(byname) for 「わするなぐさ（忘るな草）」, but nowadays 「忘れな草」(ワスレナグサ) is much more common than the original one.
According to this answer:
忘るは古語です。昔のことばです。
「忘る」 is the old form of 「忘れる」, so the negative imperative / prohibitionary な comes after the 終止形 or terminal form of the verb, naturally becoming 「忘れな」, and hence 「忘れな草」. Thanks to @Eiríkr Útlendi.
Hope this helps!
