In the taxicab Babbitt incredulously found tears crowding into his eyes.
With his head throbbing, his tongue and lips stinging from cigarettes, he incredulously counted the number of drinks he had taken, and groaned, "I got to quit!"
Incredulously he remembered that he had by his roaring familiarity with them laid himself open to the patronizing of youths whom he would have kicked out of his office; that by dancing too ardently he had exposed himself to rebukes from the rattiest of withering women.
There are no more uses of "incredulous" in the book.
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I find it incredulous that you believe she is sincere.