The "last straw" is an English and American idiomatic expression meaning the last instance in a series of events that can no longer be tolerated. Some kind of reflexive action inevitably occurs as a direct result of this insult, situation or additional burden. Paramount to grasping the full meaning of this phrase is understanding that the last straw need not be a huge change or an especially onerous task. Indeed, it is more likely trivial in nature. It is, however, this last instance in combination with all other similar ones preceding it that causes the resulting failure of tolerance, patience, civility or understanding.
Many incidents in everyday life might be said to be the "last straw." As noted above, the important aspect of the incident is that it is not necessarily worse than the chain of similar events that came before it. As a concrete example, a man might say it's the "last straw" when his neighbor's son takes a short cut through his garden for the tenth time, after he has warned him repeatedly not to do so. When this happens the final time, the man might decide to call the authorities on his neighbor as his tolerance has been exhausted.
At least two independent sources attribute this idiom to an ancient Arabic proverb in which a camel is loaded beyond its capacity to rise and carry its burden of sheaves of straw. The "last straw" is the tipping point, the one small addition that makes the load too much to bear. The story itself or even a condensed narrative, however, was not cited and cannot be located. Other variations on this idiom involve feathers or melons instead of straw, and horses, donkeys, and even monkeys as the animals that are loaded down, but the meaning remains the same.
Despite its presumably ancient origins, there is no use of the term "last straw" in the Christian Bible. A British website devoted to historical use of idioms identifies the first published appearance of "last straw" in a May 1816 edition of The Edinburgh Advertiser. The phrase later appeared in Thomas Fuller's Gnomologia. In October 1843, the expression crossed the Atlantic and appeared in The Southport American, albeit using feathers as opposed to straw. Charles Dickens used the idiom in Dombey and Son and Mark Twain used a similar one involving feathers in The Adventures of Tom Sawyer.