A very small amount or particle; the tiniest bit imaginable.
From Old English 'wiht' (a creature, thing, person), which originally meant 'a being' or 'a thing.' It specialized to mean 'a small thing' and eventually 'the smallest possible amount.'
The phrase 'not a whit' means 'not even a tiny bit'—it comes from medieval times when people would use the word 'wiht' to mean a physical creature or thing, and a small creature = a small amount!
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