Straw that broke the camel's back


The idiom "the straw that broke the camel's back", alluding to the proverb "it is the last straw that breaks the camel's back", describes the seemingly minor or routine action that causes an unpredictably large and sudden reaction, because of the cumulative effect of small actions.
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, strong reaction.

Origins and early uses

The earliest known version of the expression comes in a theological debate on causality by Thomas Hobbes and John Bramhall in 1654-1684:
An essay of 1724 emphasizes not the fact of being the last cause, but rather of being a least cause, that is, a minor one:
Attested versions of the proverb include, in chronological order:
Other variants are:
The same sentiment is also expressed by the phrase "the last drop makes the cup run over", first found in English as "When the Cup is brim full before, the last superadded drop is charged alone to be the cause of all the running over".
The image of the last drop is also found in many other languages.

Antecedents

The phrase has been compared with Seneca's discussion on why death is not to be feared. Starting with a mention of the commonplace "we do not suddenly fall on death, but advance towards it by slight degrees; we die every day", Seneca compares life to a water-clock:

It is not the last drop that empties the water-clock, but all that which previously has flowed out; similarly, the final hour when we cease to exist does not of itself bring death; it merely of itself completes the death-process. We reach death at that moment, but we have been a long time on the way.
Quemadmodum clepsydram non extremum stillicidium exhaurit, sed quicquid ante defluxit, sic ultima hora, qua esse desinimus, non sola mortem facit, sed sola consummat; tunc ad illam pervenimus, sed diu venimus.

In contrast to the imagery of the "last straw", which emphasizes dramatic final result, Seneca emphasizes the continuity of the final hour of life with all the hours that have come before.