Where does “こちらこそ” come from?

こちらこそ (Japanese) comes from Japanese こそ, from Old Japanese 此 — this.

Ancestry of “こちらこそ”, step by step

StepLanguageWordMeaning
1Japaneseこそemphatic particle used for emphasis: it is... that
2Old Japanese此this
Every word from Old Japanese 此 →Every word from Japanese こそ →
こちらこそ — Etymology, Origin & Word History | Etymology Explorer