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Well, really Einstein ! That's the last straw ! Interesting that it is literally an allusion to something being the last burden , that would not in itself be disastrous, causing a collapse and so it has come to be used sometimes for the terminal misfortune of a company. I only knew it as 'the last annoyance or frustration which has exhausted my patience' ; this is a far less obvious meaning, though certainly the only one that I was brought up with. It has an air of P G Wodehouse about it and was , indeed,favoured by my elderly aunts.