Whereas "Słowianie" ("Slavs") comes from "słowo" = "word"...
Not just an urban legend, it seems! Eastern lavic languages use the same word for Germans.
In Ukrainian there is їхній/їхня/їхнє/їхні (jichnij/jichnia/jichnie/jichni) that means "theirs," and in Russian non-standard colloquial ихний/ихняя/ихние (ichnij/ichniaja/ichnije) that is used ungrammatically for "theirs," considered incorrect but common in certain dialects. The correct form is only их (ich). Polish has no version of those?