last straw

[last straw] or [straw that breaks the camel's back] {n. phr.} Asmall trouble which follows other troubles and makes one lose patienceand be unable to bear them.

Bill had a bad day in school yesterday.He lost his knife on the way home, then he fell down, and when hebroke a shoe lace, that was the last straw and he began to cry.

Mary didn't like it when the other girls said she was proud and lazy,but when they said she told fibs it was the straw that broke thecamel's back and she told the teacher.