"cloud of bees or other insects," Old English swearm "swarm, multitude," from Proto-Germanic *swarmaz (source also of Old Saxon, Middle Low German swarm, Danish sværm "a swarm," Swedish svärm, Middle Dutch swerm, Old High German swaram, German Schwarm "swarm;" Old Norse svarmr "tumult"), by Watkins, etc., derived from PIE imitative root *swer- "to buzz, whisper" (see susurration) on notion of humming sound, and thus probably originally of bees. But OED suggests possible connection with base of swerve and ground sense of "agitated, confused, or deflected motion." General sense "large, dense throng" is from early 15c.
"to climb (a tree, pole, etc.) by clasping with the arms and legs alternately, to shin," 1540s, of uncertain origin. "Perh. orig. a sailor's word borrowed from the Continent, but no trace of the meaning has been discovered for phonetically corresponding words" [OED]. perhaps originally a sailors' word, of uncertain origin. Also recorded as swarve (16c.) and in Northern dialects swarble, swarmle. Related: Swarmed; swarming.
"to leave a hive to start another," also "to gather in a swarm, crowd, or throng," late 14c., from swarm (n.). Compare Dutch zwermen, German schwärmen, Danish sværme. Related: Swarmed; swarming.