The word boatswain has existed in one form or another longer than Modern English has (Modern English only dates back to the beginning of the Renaissance).
It is derived from late Old English batswegen, from bat ("boat") + Old Norse sveinn ("swain"), meaning a young man, a follower, retainer, or servant.
Various phonetic spellings (such as "bosun" and "Bos'n") have also been in use through the centuries.

Originally, on board sailing ships the boatswain was in charge of a ship's anchors, cordage, colors, deck crew, and the ship's boats.
The boatswain would also be in charge of the rigging while the ship was in dock.
The boatswain's technical tasks have been modernized with the advent of steam engines and subsequent mechanisation.
The word boatswain has existed in one form or another longer than Modern English has (Modern English only dates back to the beginning of the Renaissance). It is derived from late Old English batswegen, from bat ("boat") + Old Norse sveinn ("swain"), meaning a young man, a follower, retainer, or servant. Various phonetic spellings (such as "bosun" and "Bos'n") have also been in use through the centuries.
Originally, on board sailing ships the boatswain was in charge of a ship's anchors, cordage, colors, deck crew, and the ship's boats. The boatswain would also be in charge of the rigging while the ship was in dock. The boatswain's technical tasks have been modernized with the advent of steam engines and subsequent mechanisation.
Boatswain's mate (United States Navy)