Gravad lax is a dirty word. It means 'buried salmon' in Swedish. It refers to the medieval custom in Scandinavia (where the dish is more commonly called gravlax in Sweden and also gravlaks in Denmark and Norway) of burying lightly salted salmon and other types of fish in the ground or barrels covered with a bit of birch bark for either a few days (the result being similar to modern preparations of gravlax) or many months as a way of preserving fish for winter. The fermented fish produced from long-term burial was of a lovely, buttery texture, powerfully sour and, as you would imagine, quite malodorous, too. By the 18th-century gravlax was no longer fermented, notes Harold McGee in On Food and Cooking, simply salted and weighted for one to two days. It's unclear when fresh dill became the standard pairing but McGee suggests replacing dill with pine needles for a taste of the old ways.
This Scandinavian dish of salmon cured with salt and dill has underground origins in the Middle Ages.