the straw that broke the camel's back

Meanings

1. A small and seemingly insignificant addition to a burden that renders it too much to bear; the small thing which causes failure, or causes inability or unwillingness to endure any more of something.

Etymology

Possibly from the proverb, recorded by Thomas Fuller in his Gnomologia (1732) as “’Tis the last Feather, that breaks the Horse’s Back.” This comes from an Arab proverb about loading up a camel beyond its capacity to move.