the final/last straw

(also the straw that breaks the camel's back)

C1 Level
the last in a series of unpleasant events that finally makes you feel that you cannot continue to accept a bad situation:

Losing my job was bad enough, but being evicted was the final straw.

She's always been rude to me, but it was the last straw when she started insulting my mother.

Source:
https://dictionary.cambridge.org/dictionary/english/final-last-straw