Complex sentence: The German word unbesiegbar cannot be inflected, so there is no form like "unbesiegbarste"; "siegbar" is not a German word, there is little to no meaning in the German language.
Split into simpler sentences: The German word unbesiegbar cannot be inflected, so there is no form like "unbesiegbarste"; "siegbar" is not a German word. Which ever explanation is meant there is little to no meaning in the German language.