“The straw that broke the camel’s back” is an idiom also known by its abbreviated version “the last straw.”
This is a reference to a seemingly inconsequential action causing a catastrophic failure. It means that someone’s limits have been exceeded. They have reached the last in a line of inexcusable or inappropriate occurrences and can’t take any more. Example:
“Son, you’re behavior lately has been less than acceptable. Your grades are going downhill, your attitude has been negative, but the fact you stayed out all night is the straw that broke the camel’s back. Give me the keys to your car.”
Where did it come from?
This phrase originally came from an Arab proverb about a camel who was loaded with straw. His load grew and grew and grew until finally one last little straw proved too much and caused his back to break. Its most influential appearance in literature was thanks to Charles Dickens in Dombey and Son published in the 19th century:
“As the last straw breaks the laden camel’s back.”
Other variations of this idiom are: “the straw that broke the donkey‘s back”, “melon that broke the monkey‘s back”, “feather that broke the camel’s back,” “feather that broke the horses’ back, “the drop that makes the cup run over.”