The straw that broke the camel's back is the last thing you are willing to accept after which you will not put up with any more. Example: "You broke a lot of rules and we tried to ignore it. But stealing money from us was the straw that broke the camel's back. We're sending you home to your parents."

You are at the last straw when you are just about to lose your patience. The straw that broke the camel's back is the last thing that that happened that finally made you upset. Example: "He came to work late every day that week. Then on Friday, he didn't show up at all." Reply: "That was the straw that broke the camel's back."

There is a limit to how much straw (long yellow grass) a camel can carry on its back. If you keep putting more straw on top, it will finally break the camel's back. When you are at the last straw you are finally angry and will not take any more. Example: "You have been rude to me all day. I've had it. That's the last straw!"