grammar - Meaning and usage of ~としたことが

I came across the expression ~としたことが, for example:

私としたことが

I'm trying to figure out the meaning and usage of this phrase, but I can't find any English resources that explain it.

What does this mean?

Answer

としたことが and ともあろうものが are used to express the surprise of the speaker toward the (bad) behaviour of someone.

With 私, it expresses something around the line of "Who could have thought I/someone like me/someone of my standing/someone of my position (would do such a thing)"

Here are some examples from the 和英大辞典：

君としたことが, とんだへまをしでかしてくれたものだ.　

You, of all people, have made an awful blunder.

あらあら私としたことが.　

Oh my, look what I've done.

彼女としたことが, そこでついつい眠り込んでしまったらしい.　

Then she, would you believe it, seems to have fallen sound asleep.