Consider:

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> ‘Why what else are you?’ returned John, looking down upon her with a smile, and giving her waist as light a squeeze as his huge hand and arm could give. ‘A dot and’—here he glanced at the baby—‘*a dot and carry*—I won’t say it, for fear I should spoil it; but I was very near a joke. I don’t know as ever I was nearer.’
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What does 'a dot and carry' mean in this context?