The last of a series of annoyances or disappointments that leads one to a final loss of patience, temper, trust, or hope.
The last of a sequence of annoyances or troubles that results in a breakdown, loss of patience, etc.
(slang) A small addition to a burden which causes it to exceed the capacity.
That's the last straw; it's a petty demand but I'm already under too much work. I quit!
Origin of last-straw
From the proverb “It's the last straw that breaks the camel's back”