In Dutch we say aardappel (earth apple; means potato) to stupid people, not Germans in particular as far as I knew. It might originate from that, I don't know.
> In Dutch we say aardappel (earth apple; means potato) to stupid people
Same in France with "patate". Funny to notice that the more common French expression for "potato" is "pomme de terre" ("apple of earth"), just like in Dutch!
Though in the south (both south NL and Flemish Belgium) they're called "friet", as in "gefrituurd" ("fries" as in "they're fried"). Still, patat is officially Dutch and according to some website 95% of the Flemish people and 100% of the Dutch people know the word.
May be regional, I'm not sure. Might have picked it from in either Limburg, Noord-Brabant or Gelderland. Pretty sure though that it's fairly universal, even if it's not something lots of people say every day.
Same in France with "patate". Funny to notice that the more common French expression for "potato" is "pomme de terre" ("apple of earth"), just like in Dutch!