A famous story describes a camel whose back was broken when it was used to carry too much straw. The last straw is used to let someone know that a behavior is not acceptable and that you will no longer tolerate it.
真不敢相信，你竟然再一次欺騙我；這可是最後一次了！
還記得「壓到駱駝的最後一根稻草」那則故事嗎？句中 the last straw 就是那最後一根稻草，用來警告對方：「我已忍無可忍，這是你最後一次耍我了。」