When a sailor (or officer) was changing permanent duty stations and had to move, they were allowed to take up to three months' pay in advance to help cover the incidental expenses of the move. Many of those costs would be later paid back after a claim was submitted, but by having the cash on hand, people could avoid having to pay credit card interest (if they even had one, back then).
The catch, of course, was that you had to pay it all back in a year, which was taken out of your pay. If you took the full amount, you then had a 25% pay cut for the next year.
The story back then was that you were paying for something you no longer could use, so it was like making payments on a dead horse. But we didn't have the internet back then, so who knew that the origin was somethingelse entirely?