What does "a baker's dozen" mean — and why is it funny?

informal

Meaning

Thirteen, rather than the usual twelve.

Where it comes from

From medieval England, where bakers caught selling underweight bread faced harsh punishment, so they added a spare loaf to every dozen to be safe.

Why it is funny

The quiet humor is the nervous over-honesty behind it. An entire counting word exists purely because bakers were once so frightened of the law that they gave away extra bread out of sheer self-preservation.

Used in a sentence

"She brought a baker's dozen of muffins — one each, plus a spare."