What does "tenable" mean?

Tenable: Capable of being held, naintained, or defended, as against an assailant or objector, or againts attempts to take or process; as, a tenable fortress, a tenable argument. If you have hitherto concealed his sight, Let it be tenable in your silence still. Shak. I would be the last man in the world to give up his cause when it was tenable. Sir W. Scott.