boatswain (n.) Look up boatswain at
mid-15c., from late Old English batswegen, from bat "boat" (see boat (n.)) + Old Norse sveinn "boy" (see swain). Phonetic spelling bo'sun/bosun is attested from 1840.
BOATSWAIN. The warrant officer who in the old Navy was responsible for all the gear that set the ship in motion and all the tackle that kept her at rest. [Sir Geoffrey Callender, "Sea Passages," 1943]