Fig.: no matter what happens

When school is out, come hell or high water, I'm going on vacation for a whole month

Finita la scuola, caschi il mondo, vado in vacanza per un mese intero

I don't care if the world comes to an end tomorrow: tonight you'll be home at 10:00 or you'll be in trouble

Cascasse il mondo, questa sera sarai a casa alle 10:00 o saranno guai

At 21 Mary finally decided to leave her family, come hell or high waters, and make a life of her own

A 21 anni Maria ha finalmente deciso di uscire di casa e di andare a vivere da sola, cascasse il mondo

Originally, it could be a cowboy expression; a legacy of the cattle trail, when cowboys, overcoming difficulties of any kind such as high water at every river and continuous hell in between rivers, drew their cattle home