This is the 8318th most frequent Japanese word.
当の
"当の" means "the very" or "the specific."
当の本人: The phrase refers specifically to the person being discussed or considered in this context.
その問題について言えば、当の本人が一番困惑している。
Speaking of that matter, the person in question is the most perplexed.
当の主題: The word emphasizes the specific topic being referred to from the previous discussion.
昨日のパーティーで話していた当の主題をもう一度考えよう。
Let's reconsider the very topic we were discussing at yesterday's party.
当の彼: This usage highlights the exact person whose presence is pivotal in this setting.