The Last Straw

The straw that broke the camel's back is an idiom known to humanity worldwide.  Other languages often refer to a similar expression, "The drop that made the cup overflow."  The drop in a cup is a simpler experiment to reproduce, since very few of us have camels and large supplies of straw.  (Related, see also Half Empty or Half Full)

Shortened to, "That's the last straw!" the expression in American English usually refers to the final event that causes a failure, a breakdown.  Often it is thought of as the last of many events that build up and then breaks one's will or patience, causing an emotional reaction of anger or resignation.  At this point, the individual has "had enough" and is ready to give up on the problem.