the.com/final straw

means The last in a series of annoyances or problems that, though minor on its own, causes someone to finally lose patience or reach a breaking point.

from Shortened from 'the straw that broke the camel's back,' an old proverb picturing a camel loaded straw by straw until one final, utterly unremarkable piece of straw is enough to collapse it. The image appears in English writing by the 1800s (earlier versions used other last-ounce metaphors), and it endures because it captures something true: cumulative weight, not the final cause alone, does the damage.