Yep, its short for Boatswain ( better known as “Bosun’ mate”). I served with many a crusty SeaDog in the Deck Divisions of my sea commands, and there wasn’t a bad one in the lot. And they ALWAYS let the Junior Officers know what was right and wrong, along with having the CO and XO’s ear as well. If the Officers were worth their salt, they’d listen to what he’d say.
I'm reminded of the time I saw a wet-behind-the-ears butter bar ensign "remind" a crusty old BMCM (Master Chief Boatswain's Mate) that he was supposed to salute an officer when they pass each other. The BMCM's reply? He reached into his pocket and flipped a quarter to the young ensign and said, "Here boy, call your mommma and tell her you met a REAL sailor today."
Those of us who saw it fell out laughing while the poor young ensign slunk away with a red face.
Mastercard says PRICELESS!!!