1. n. The last of a series (as of events or indignities) that brings one beyond the point of endurance. From the proverb "It's the last straw that breaks the camel's back."
  2. n. (idiom) The final annoyance, disappointment, misfortune or setback, which even though minor, makes one lose patience, temper, trust or hope. Also: last straw, the final straw
Source information: Merriam-Webster, 1848.