last straw

the straw that broke the camel's back, insignificant matter that upsets a delicate balance, small issue following a series of difficult matters that causes a person to lose his cool

The idiom the straw that broke the camel's back is from an Arab proverb about loading up a camel 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 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".
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Noun
1. the final irritation that stretches you patience byond the limit
(hypernym) aggravation, irritation, provocation

(See the last straw)

Synonyms and related words:
causa causans, causa finalis, causal sequence, causing cause, efficient cause, final cause, first cause, formal cause, immanent cause, immediate cause, occasional cause, provocation, proximate cause, remote cause, stand, the last straw, the limit, transient cause, ultimate cause