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.