Two-verb compounds: 振る舞う: plain-form + plain-form verb instead of masu-form + plain form?

Question

I finished Core 10000 (an Anki deck) and 振る舞う (to entertain) was one of the last ones to pop up.

I find it interesting because to me it appears to be a combination of plain form 振る (to shake) + another plain form verb 舞う (to dance). This differs from all the other two-verb compounds I am used to such as 取り引き (transaction, noun in this case)、書き直す (rewrite)、やり直す (redo)、吹き出す (blow out), 話し出す (start talking)、飛び上がる (fly up), where the first verb is the masu-form instead of plain.

I cannot recall any similar plain-form + plain-form verbs like 振る舞う.

My theory is that either this verb is an exception to the rule or this verb is in fact, NOT a two-verb compound. It is simply ふるまう, a single verb, and the Kanji was conveniently added later to make it look like a two-Kanji compound.

Answer 1

You are correct, linking two verb in their 連体形 ("dictionary") form is unusual and unexpected.

Both of the dictionaries 日本国語大辞典 and 広辞苑 suggest the following explanation:

「観智院本名義抄」や「色葉字類抄」で「翔」を「ふるまふ」と読んでいることから「羽を存分に振って自由舞う」という「振るい舞う」が原義かとおもわれる。

That is, it is a shortening of the original form huruimau, in the sense of a bird flapping its wings and taking flight, fluttering about in the skies (= "dancing", the Japanese mau is used in this sense). hurui is the 連用形 ("masu-stem") of the verb huruu, and this explanation would render the compound regular.

However, both dictionaries imply that this is only a hypothesis, and a dictionaries main purpose is not to list etymological research. It is just as well that the kanji are simple ateji and that hurumau derives from some different word(s), or that it is simply irregular. The earliest usage a quick dictionary lookup yields is from about 800 years ago, in the 古今著聞集.