It has no meaning I have ever heard other than "off the shelf", "ready to wear", or "of a standardized dimension". I don't know of any maritime usage. I would go back to the author of this idea and ask for an example or reference.

It's possible he was thinking of peg legs, which are often associated with pirates or Captain Ahab of the Pequod, but that would be more a matter of "on the peg" than off it! :>)

A