nonplussed

Unsure how to act or respond; bewildered, perplexed.

[N]ote the honeſt Quaker vvas nonpluſſed, and greatly ſurprized at that Queſtion.

I could not discern what she meant, and I would not ask her: I was nonplussed.

Unaffected, unfazed; unimpressed.

"I regard Dean [W. P.] Baddeley's gambling activities with embarrassment and dismay," said Anglican dean of Melbourne S. Barton Babbage. […] The Rev. Baddeley remained nonplussed. "I don't intend to make a habit of going to the races but I feel clergymen should mix as our Lord did with all walks of life," Dr. Baddeley said.

And while many of us might be a little taken aback if Mom showed up at our offices, Secrist is utterly nonplussed, even happy about it.