''Leadbottom'' Navy slang or just TV show coinage?

Factual Questions
1

Background:

In the old TV series “McHale’s Navy,” Lt. Commander McHale and his band of “cutthroats” (as the captain called them) referred to Captain Wallace Burton “Jolly Wallly” Binghamton as “Ole Leadbottom.” McHale had previously been a tramp steamer skipper, but when the war broke out, he was given command of a PT boat because “he knew the area [Pacific Islands] like the back of his hand.” Wally Binghamton had never skippered anything. In civilian life, he had just been the “manager” (or whatever) of the San Diego Yacht Club, a job he so longed to return to.

Anyway, I can see the reason for calling him “Leadbottom,” but was this normal Navy slang for civilians given a command, or just a name the writers made up for the series.

2

The RandomHouse Dictionary of Historical Slang doesn’t show an entry for “leadbottom” or any variant.

It does for “ironbutt” etc.

Made up, I think.

NOt that someone couldn’t have used it in WWII, but not common.

3

It’s just slang for anyone that sits behind a desk all the time.

4

Nothing found in the etymology sources of which I’m aware. That said, I wonder if there is a tie to the practice of placing heavy items inside the keel of a vessel to improve stability, e.g. lead in the bottom. If this is true, then the connotation of ‘something in the lowest regions’ or ‘otherwise useless weight’ would hold true.