The last straw that broke the camel's back - A person can't bear one more problem or  difficulty.

Don't push him too far or the last straw may break the camel's back. 不要逼他逼得太厉害，否则他会受不了的。

After the heated argument , the wife knew it was the last straw. 经过一场激烈争吵后，太太已经知道无法再忍受了。