A loud and confusing noise or to make such a noise; can also mean insistent public demand or outcry.
From Old French 'clamor' meaning 'outcry,' derived from Latin 'clamor' (shouting, noise). The spelling with 'u' is British; the meaning evolved from literal noise to metaphorical noisy demands.
Medieval town criers used to make clamour to gather crowds—the word literally captures the sound of chaos and collective human voices demanding attention, which is why we still say people 'clamour for' change or justice.
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