ぞ is the short form of こそ?

Question

Can ぞ be the short form of こそ? Here below a sentence example:

これぞあたしの本だ！！

Here I found that これぞ = this (emphatic), but I'm curious to know if that zo came from koso or not. Thk U in advance!

Answer 1

If ぞ and こそ are etymologically related to one another, it will have been a long time ago. Both particles have a long history in the Japanese language and were used commonly in Classical Japanese. However, it seems more likely that こそ is derived from ぞ rather than the other way around.

In Classical Japanese, the two particles actually functioned differently on a grammatical level - Classical Japanese had a feature called 係り{かかり}結び{むすび}, in which certain particles caused the following verb to take a different form, and the forms used were different for ぞ (which used the 連用形{れんようけい}) and こそ (which used the 已然形{いぜんけい}). So they were clearly quite distinct particles as long ago as Classical Japanese.

That said, ぞ was apparently pronounced as そ earlier in the language's history (the Wikipedia article for 係り結び mentions that it was そ in the 上代{じょうだい} period), so it's not unlikely that the two words are related if you go back far enough. The Wiktionary entry for the classical こそ mentions that one theory is that こそ comes from ぞ combined with the classical pronoun こ, which would make こそ originally equivalent to the whole expression これぞ itself!

Answer 2

This ぞ is an archaic Japanese particle which works similarly to こそ in modern Japanese. In modern Japanese, これぞ is not common any more, but これぞＸ（だ）！ is sometimes used like a catchphrase:

• これぞラーメン！ "This is (true) ramen!"
• これぞアメリカだ！ "This is America!"

Besides, これぞ is occasionally used to simulate old-fashioned, pompous and/or chūnibyo-like way of speaking, mainly in fiction. 「これぞあたしの本だ！！」 is not incorrect, but sounds a little funny because the これぞ part is unnaturally archaic.

Something like this would be fine in a fantasy work (imagine an old wizard with long beard saying this):

これぞ吾輩が長年探し求めし魔道書！！

No one speak like this in reality, though.