By the way; happening by chance rather than being planned or essential; as a minor point.
From the adjective 'incidental,' which comes from Latin 'incidere' (to happen to), with the adverbial suffix '-ly.' The prefix 'in-' plus 'cadere' (to fall) originally meant 'falling upon' or 'happening to.'
When someone says 'incidentally' they're supposedly mentioning something minor, but that's often when the most important information comes out—it's a psychological trick called the illusory truth effect.
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