Etymology
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boatswain (n.)

mid-15c., from late Old English batswegen, from bat "boat" (see boat (n.)) + Old Norse sveinn "boy" (see swain).

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]

He also summons the hands to their duties with a silver whistle. Phonetic spelling bo'sun/bosun is attested from 1840. Fowler [1926] writes, "The nautical pronunciation (bō'sn) has become so general that to avoid it is more affected than to use it."

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bosun (n.)
a mid-19c. respelling to reflect the modern pronunciation of boatswain.
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*bheid- 
Proto-Indo-European root meaning "to split," with derivatives in Germanic "referring to biting (hence also to eating and to hunting) and woodworking" [Watkins].

It forms all or part of: abet; bait (n.) "food used to attract prey;" bait (v.) "to torment, persecute;" bateau; beetle (n.1) "type of insect; bit (n.1) "small piece;" bite; bitter; bitter end; boat; boatswain; -fid; fissile; fission; fissure; giblets; pita; pizza; vent (n.).

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit bhinadmi "I cleave," Latin Latin findere "to split, cleave, separate, divide," Old High German bizzan "to bite," Old English bita "a piece bitten off, morsel," Old Norse beita "to hunt with dogs," beita "pasture, food."
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