: nevertheless, notwithstanding
You can spell it natheless or nathless. It's been in use (in various forms) for almost a thousand years, and comes from the Old English phrase nā thē lǣs, meaning "not the less."
Natheless, while such and so preposterous were the opinions on either side, there were, it cannot be doubted, men of virtue and worth on both, to entitle either party to claim merit from its martyrs.
— Walter Scott, The Heart of Mid-Lothian, 1818