I remember reading somewhere that there is a word in old Norse for unwise/foolish that etymologically refers to someone who's not travelled or left home. Any ideas?
You are probably thinking of Icelandic heimskur "stupid, foolish", which comes from Old Norse heimskr "foolish, silly; who has never been from home" (from ON heima "home"). It appears frequently in Norse sagas, sometimes in the intensive form allheimskr "very stupid". A famous instance of this is the saying "Engi er allheimskr, ef þegja má" ("No one is a total fool, if one knows when to be silent") in the Saga of Grettir the Strong.
I am not aware of an opposite word relating wisdom with being well-travelled, though.
1There's Ingvar the Far-Travelled, (Old Norse: Yngvarr víðförli,) but the name is indeed not associated with being wise, as his entire army was eradicated on his last journey.– CodosaurMay 5, 2021 at 20:00