こそ

From Old Japanese.

Possibly derived from 此 (ko, “this”) + そ (so, Old Japanese emphatic particle). (Can this⁽⁺⁾ etymology be sourced?)

こそ • (koso) 

1. emphatic particle used for emphasis: it is ... that

   今日(きょう)こそ、やるぞ！

   Kyō koso, yaru zo!

   Today is the day I'm gonna do it!

   君(きみ)が好(す)きだからこそこれほど頑(がん)張(ば)っているんだよ。

   Kimi ga suki da kara koso kore hodo ganbatte iru n da yo.

   It's because I like you [because of my fondness for you] that I'm working this hard.

   1. used in set phrases when returning a greeting or apology to someone

      「ありがとうございます！」「こちらこそ！」

      “Arigatō gozaimasu!” “Kochira koso!”

      "Thank you very much!" "Thank you, too!" (lit. "It is me who should say thank you.")

• こそ cannot be after the predicate of a sentence. Thus, when emphasizing predicates, the structure "verbal noun + こそ + する (suru)" must be used.

  賛(さん)成(せい)こそするが、積(せっ)極(きょく)的(てき)ではない

  sansei koso suru ga, sekkyokuteki de wa nai

  Although I do not oppose, I don't strongly support it either.

• There is no direct translation, but it is roughly analogous to “precisely” or “the one that”. Compare the Ancient Greek discourse particle γε (ge).

• それこそ (sorekoso)
• ばこそ (-ba koso)
• もこそ (mo koso)

こそ • (-koso) 

1. (archaic) used for calling or appealing

Possibly derived from 此 (ko₂, “this”) +‎ そ (so₂, empathic particle). (Can this⁽⁺⁾ etymology be sourced?)

こそ (ko₂so₂)

1. emphatic particle used for emphasis
2. sentence-ending particle expressing humility
3. sentence-ending particle expressing hope for others

• Japanese: こそ (koso)