Huayhuash is the name of a chain of mountains ("Cordillera") next to Cordillera Blanca in Peru. The Huayhuash trek takes 8 to 12 days to complete (depending on the route taken and fitness of its trekkers), and is commonly referred as "Why wash?" as there are no bathroom facilities throughout the entire length of the trek, thus you can't wash yourself for at least 8 or 9 days.
You can bathe in hot springs once in the middle of the trek, but it is forbidden to use shampoos, soaps, or other chemical products to wash yourself in order to avoid pollution in the natural water of the park.
Adventurer: Why wash in Huayhuash?
Tourist: What? Speak English!