If you increase a camel’s burden straw by straw, eventually you will load him one straw too many that his back will be broken. The phrase ‘the last straw’ refers to something that, although small in itself, comes after other troublesome things and produces at last a feeling of being intolerable.
Mr Jackson’s neighbours had very noisy children. He put up with it as long as hi could, but the last straw was when the boy next door decided to learn to play the bugle.