Pronounced bosun. Originally the person responsible for discipline on a ship. Now the position of one who works with line or rope, still a very common position in the navies of most countries. Sailors were considered to be the originators of many knots still in use today. A boatswain's assistant is known as a boatswain's mate.
A keen boatswain will be able to show you how to tie a sheet bend, a Spanish bowline, a carrick bend, and many other kinds of knots if you ask him.