the straw that broke the camel's back

The final limit of capacity, including patience. An Arabian anecdote told of a camel whose owner loaded the beast of burden with as much straw as possible. Not satisfied with the staggering load he had put on the camel, the owner added just one last piece of straw. Even that one wisp was too much, and the animal collapsed with a broken back, leaving the owner with no way to take his goods to the market. The story is a parable for all the times you've been repeatedly irked until you can't take it anymore and you explode.