As Marilla watched the bright, animated face and graceful motions her thoughts went back to the evening Anne had arrived at Green Gables, and memory recalled a vivid picture of the odd, frightened child in her preposterous yellowish-brown wincey dress, the heartbreak looking out of her tearful eyes.
There are no more uses of "preposterous" in the book.
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They can’t be seriously considering such a preposterous idea.
a preposterous attempt to turn back the pages of history