at the end of one's tether. At the limits of one's resources, abilities, endurance, or patience. For example, If that loan doesn't come through, we'll be at the end of our rope, or The workmen are driving me crazy; I'm at the end of my tether. This expression alludes to a tied-up animal that can graze only as far as the rope (or tether) permits.
[ Late 1600s