the last straw
Meaning: Something is the last straw if it's the latest in a series of annoying or upsetting events, and it's the one that finally makes you do something about the situation.
- I was already close to resigning, but the last straw was being bossed around by some kid twenty years younger than me. After that, I quit the job.
- I saw my girlfriend flirting with another guy at the party, and that was the last straw. I told her our relationship was over and I left.