In later years, I sometimes wondered exactly what made Jem do it, what made him break the bonds of "You just be a gentleman, son," and the phase of self-conscious rectitude he had recently entered.
There are no more uses of "rectitude" in the book.
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Mr. Cunningham, a man of uncompromising rectitude, had given her a pint free of charge for having guessed his name yesterday, one of the tiny things she adored about Maycomb: people remembered their promises.
Harper Lee -- Go Set a Watchman
Now I sat down to write a different kind of letter, one that tried to convey how much she meant to me, how much she had taught me, how I wanted to emulate her rigor and rectitude, how much I loved her and missed her.