Old English clyster "a number of things growing naturally together," probably from the same root as clot (n.). Meaning "a number of persons, animals, or things gathered in a close body" is from c. 1400. Of stars, from 1727. Cluster-bomb attested by 1950.
late 14c. (transitive), "to collect into a cluster or group," from cluster (n.). Intransitive sense, "to form or constitute a cluster," is from 1540s. Related: Clustered; clustering.