According to <https://www.dict.cc/?s=achter>, not only is it northern german, but also "sailor language"! :D English is similar, having "aft" for sailors to say "rear". I know boat people have specialized terms but never thought of it as sailor speak but rather like any other jargon. For some odd reason, the dictcc page also mentions the random fun fact that "achter" is Afrikaans, ignoring where Afrikaans got it from (or rather, who forced it upon them)