the straw that broke the camel's back

Possibly from the proverb, recorded by Thomas Fuller in his Gnomologia 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.

the straw that broke the camel’s back

1. (idiomatic) 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.
   • 1848, Charles Dickens, Dombey and Son:

     As the last straw breaks the laden camel's back.

• final straw
• last straw