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"cloud of bees or other insects," Old English swearm, from Proto-Germanic *swarmaz (cf. Old Saxon, Middle Low German swarm, Swedish svärm, Middle Dutch swerm, Old High German swarm, German Schwarm "swarm;" Old Norse svarmr "tumult"), usually derived from PIE imitative root *swer- (2) "to buzz, whisper" (see susurration) on notion of humming sound. But OED suggests possible connection with base of swerve and ground sense of "agitated, confused, or deflected motion."
"to climb by clasping with the arms and legs alternately," 1540s, perhaps originally a sailors' word, of uncertain origin. Also recorded as swarve (16c.) and in Northern dialects swarble, swarmle.
"to leave a hive to start another," late 14c., from swarm (n.). Related: Swarmed; swarming.