It’s fascinating to read it so aptly described. I thought it was obvious what the saying meant. That’s why it is usually said to express exasperation. When someone has already endured a lot of abuse, it takes only but a minor inconvenience or minimal annoyance to finally drive them beyond the treshold of tolerance.

That’s the image, even of such an insignificant weight as that of a straw is capable of breaking someone if they have been overburdened for long already.