When you keep trying to reach the limit of what is possible, there are several potential outcomes. If you are lucky, you might succeed at your endeavour and everything is well. However, the opposite is just as likely. Sooner or later you might reach critical mass, a point of no return. This is the last straw.
This idiom means that one additional relatively small occurrence ultimately leads to the collapse of the whole system. This expression originates from the proverb “it is the last straw that breaks the camel’s back”. It has nothing to do with the plastic straws we used until last year, or the metal/glass/paper straws we use today. The carrying capacity of the camel was at its peak. Straw weighs next to nothing but it was the last straw that ultimately broke the camel’s back.
A topical example is the racism people of colour have to face every day. The institutional racism in the US especially (although Germany certainly has its own issues with racism, which need to be addressed) has been one cause of discrimination against people of colour, and the latest proverbial “last straw” was the murder of George Floyd by the police. People had had enough and the camel’s back was broken.
