Etymology of もん・もの

Question

How do I make sense of the 終助詞 もん as in

おいしいもん
そうなんだもん

Although I have only heard it in 時代劇 speak, I guess it comes from もの, which I think should be も + の.

But what も can follow the 終止形 and what の can follow that も. Is も the same も that roughly means "also"? Is の the nominalizer の here?

P.S. I am not asking what it means, when it is used, etc. I want to know how to make sense of it grammatically. At the moment, it has also been suggested that もん・もの derived from 物, but it seems to follow different rules than what one would expect of a particle derived from a noun.

Answer 1

If I ignore semantics and discuss syntax only, I think that it went like this:

1. もの was originally 物, a lexical noun (実質名詞), which could be modified by a relative clause ending in 用言の連体形.
2. It was then grammaticalized into a formal noun (形式名詞), losing its literal meaning but still appearing in the same syntactic position, after a relative clause ending in 用言の連体形.
3. After the morphophonological merger of 連体形 and 終止形, it was possible to reanalyze 連体形＋もの as 終止形＋もの because the two looked and sounded the same. This in turn made it possible to reanalyze もの as a 終助詞.
4. Once it was possible for もの to be treated as a 終助詞, it became possible for it to appear after だ.

So now もの (or its contracted form もん) can appear as a 形式名詞 or a 終助詞. And of course the original word 物 is still around.

Answer 2

According to the book I have here "All about particles" (Chino, 1991), もの is one of the sentence final particles and its primary meaning is "because" or "reason is." Chino says もの "indicates an excuse, a dissatisfaction, or a desire to be indulged or pampared." I believe もん has the same meaning but used by mainly children and women in casual conversations.

Plain(short) form + もん Often used with んだ.

宿題したくなーい。むずかしいんだもん。

ジェットコースターは乗りたくない・・・。こわいんだもん。

えー買ってくれないの？！じゃあ、いいもん。自分で買うもん。だってどうしてもほしいんだもん！

お母さん：そんなお菓子食べて・・・。おいしいの？

子ども：おいしいもん！（怒）お母さんにはわかんないよ！

Ａ：今日夕飯いらない。

女：えー、一緒に食べようと思って、待って（い）たのにー！もーいいよ！一人で食べるもん！（怒）