What does "the straw that broke the camel's back" mean — and why is it funny?

informal

Meaning

The final small problem that, added to many others, finally causes a collapse or breakdown.

Where it comes from

An old proverb. A camel can carry an enormous load, until one last, near-weightless straw tips it past its limit.

Why it is funny

The humor is the absurd unfairness of the image. After a mountain of heavy burdens, it is one single, feather-light straw that gets the blame for the whole disaster.

Used in a sentence

"The broken printer was the straw that broke the camel's back, and she quit that afternoon."