Look up the straw that broke the camel's back in Wiktionary, the free dictionary.
The idiom "the straw that broke the camel's back" describes a minor or routine action that causes an unpredictably large and sudden reaction due to the cumulative effect of previous small actions. It alludes to the proverb "it is the last straw that breaks the camel's back".
This gives rise to the phrase "the last straw", or "the final straw", meaning that the last one in a line of unacceptable occurrences causes a seemingly sudden and strong reaction.