The poor soul was poring hard over a tattered book, with the traces of recent tears still upon his face; vainly endeavouring to master some task which a child of nine years old, possessed of ordinary powers, could have conquered with ease, but which, to the addled brain of the crushed boy of nineteen, was a sealed and hopeless mystery.
There are no more uses of "addle" in the book.
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But she counted and counted till she got that addled she’d start to count in the basket for a spoon sometimes; and so, three times they come out right, and three times they come out wrong.
Mark Twain -- The Adventures of Huckleberry Finn
Although her relations with Atticus and Jem had reached their lowest ebb ("You’re downright addled these days, Jean Louise," her father had said.