A historic term used in the merchant service and early navies to describe an unskilled, fee-paying apprentice or a very junior midshipman. These individuals were often placed on board through family connections rather than merit, lacking the practical seamanship skills of the working crew. The term highlights their status as an experimental or unproven element on the deck.
Heard on the job
Don't let the guinea pig handle the halyard alone, he'll foul the lines before we even clear the harbor.