expression
of all people; even someone like ...
Used to express surprise or disappointment that someone (often oneself or someone reliable) did something unexpected or made a mistake. Follows a noun or pronoun, e.g. 私としたことが, 彼としたことが.
私としたことが、こんな簡単なミスをするなんて。
Of all people, I can't believe I made such a simple mistake.
彼としたことが、約束を忘れるとは思わなかった。
I never thought that he, of all people, would forget a promise.