Hold Onto the Meaning of Tenable

Tenable means "holdable". In the past it was often used in a physical sense—for example, to refer to a city that an army was trying to "hold" militarily against an enemy force. But nowadays it's almost always used when speaking of "held" ideas and theories. If you hold an opinion but evidence appears that completely contradicts it, your opinion is no longer tenable. So, for example, the old ideas that cancer is infectious or that being bled by leeches can cure your whooping cough now seem untenable.