Note: In this phrase, by seems to be used in the sense of nearness in time, and to be repeated for the sake of emphasis, and thus to be equivalent to "soon, and soon," that is instantly; hence, -- less emphatically, -- pretty soon, presently.
We shall understand one another by and by. I have always got on with ladies
Gustave Flaubert. Madame Bovary
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“Don’t come bothering me with your little troubles, boy. You’ll git something bye and bye”—when I would try to apologize for not paying my rent and board.
Ralph Ellison. The Invisible Man (1952)
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By and by I was close enough to have a look,
Mark Twain. Huckleberry Finn
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now that there was no mistress; and as to Hareton, the curate should take him in hand, by-and-by.
Emily Brontë. Wuthering Heights (1847)
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Where is my Subtle, there? Within, ho!
FACE [within]. Sir, he'll come to you by and by.
MAM. That is his fire-drake,
His Lungs, his Zephyrus, he that puffs his coals,
Ben Jonson. The Alchemist (1610)
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Perhaps by and by I may observe that private balls are much pleasanter than public ones. But now we may be silent."
Jane Austen. Pride and Prejudice (1813)
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I had filled up the bottle from the tar-water jug. I knew he would be worse by and by.