Making a sharp, repeated tapping sound, usually on a door to get someone's attention; the sound itself.
From Old English 'cnucian' with uncertain ultimate origin, possibly imitative. The 'kn' spelling comes from Old English but the 'k' became silent in Middle English, a pattern seen in 'knife' and 'know.'
The silent 'k' in 'knocking' is a fossil from Middle English: people stopped pronouncing it around the 1600s, but writers kept spelling it that way out of habit—so we're literally writing the pronunciation of a 400-year-old sound.
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