Sometimes shortened to the last straw.
= the last bad thing (even a small one) in a series of bad things that finally makes someone very angry and/or take action
Literal idea: a camel carrying a lot of hay; one more small straw finally breaks its back.
Ex: I've had a bad week, but the bird pooping on my head was the straw that broke the camel's back.
Ex: After making many obscene phone calls, he later threatened her son. For her, that was the straw that broke the camel's back, and she called the police.