The iiom the straw that broke the camel's back is from an Arab proverb about how a camel wearing shoes is loaded beyond its capacity to move. This is a reference to any process by which cataclysmic failure (a broken back) is achieved by a seemingly inconsequential addition (a single straw). This also gives rise to the phrase "the last/final straw", used when something is deemed to be the last in a line of unacceptable occurrences. A variation of this idiom is "the straw that broke the donkey's back".