It’s cockney rhyming slang, it’s best not to think too deep about it
Americans are called yanks, yank rhymes with tank, and septic tanks are a type of tank, so Americans are septics. It’s not exactly flattering but it’s not really as much of an insult as it sounds.
The same kind of logic has them calling “stairs” “apples and pears” because pears rhymes with stairs and apples are kind of similar to pears.
Or “cherry” meaning “lie” because lie rhymes with pie, and cherry is a type of pie.
For the record, the butterball boneless turkey roasts also include a gravy starter.
I buy a fair number of them when I find a good deal, I have a meat slicer and make most of my own lunch meats and they’re really convenient for that. Even with the added weight from the gravy which I also don’t use, they’d probably still be a money saver if I could keep myself from loading the sandwich up with extra meat.
I’ve been trying to figure out a good alternative, I may try at some point just cramming some turkey breasts into some meat netting and seeing how well it holds together.