Hi, This work by Cabell is a magical / fantastical one. I suspect this is just a nonsense word, although the author does like to throw in odd and obscure references. Clive
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It's a general exclamation of incredulity, like "good grief" or "sweet fancy Moses". Domnei isn't the only of Cabell's books this word appears in, and Cabell isn't the only author to use it- I've seen it used in Harper's magazine a few times.
I can't say how much is coincidence, but every situation I've seen it in, it's been early 20th century American literature set in feudal Eu