Honestly, that's a really good question.
In short, I don't know. It probably dated back to the 1960's and probably had something to do with the way clothes smell in an amine environment. Or, it might be due to submarine humor and how tough it is to unzip and drop trow when nature calls? They unzip just fine but it can be a struggle to get it past your shoulders.
It might have been an "A Gang" thing? I know MM's working on piping systems on skimmers favored wearing them if they were working in the bilges.
I heard my share of the traditional Navy non-sequesters and scuttle butt, but poopie suits were always just poopie suits. I never heard an actual reason why they got the name.
BTW, were you on my qual board back in 1989? That was a really good and totally unexpected question.
