Tom bent down close to it and said, "Lady-bug, lady-bug, fly away home, your house is on fire, your children’s alone," and she took wing and went off to see about it —which did not surprise the boy, for he knew of old that this insect was credulous ... and he had practised upon its simplicity more than once.
There are no more uses of "credulous" in the book.
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The trick would fool none but the most credulous.
She is so credulous she believes everything she reads.