Wiktionary
untenably
adv. In an untenable way.
Usage examples of "untenably".
But it was a ridiculous nonsense, for when I had paddled to the derelict it was only to be thrown into paroxysms of rage by repeated failures to scale her bulwarks, low as they were: for though my hands could easily reach, I could find no hold on the slimy mass, and three rope-ends which I seized were also untenably slippery, so that I collapsed always back into the boat, my clothes a mass of filth, and the only thought in my blazing brain a twenty-pound charge of guncotton, of which I had plenty, to blast her backside to uttermost Hell.