to gather or crowd together in a cluster or group.
From 'be-' prefix plus 'cluster'. This likely emerged in Middle English as one of many 'be-' verbs created to mean 'to cause to become' or 'to gather into' clusters.
This word represents the productive creativity of English—someone needed to say 'to form clusters' and just... made it by combining prefixes and roots, which is still how English speakers coin new words today.
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