So this greenhorn asked the oldest tar what the words mean. And the old tar is remembering from when he was a novice. Or he doesn't know and tries to guess at the origin. Or wants to shut the newbie up. Memory plays tricks - it is not to be relied on unconditionally.

Stands to reason that the old lag, in his invention, will relate it to his profession. And maybe the old lag is having a joke at the expense of the naive. THAT is a tradition as old as the hills. Ask anyone who served as an apprentice.

So all explanations are valid, in the context of the route they take to get to this century.

And shanty double and triple entendre is pretty universal. Plenty of examples.