What does "mutton dressed as lamb" mean — and why is it funny?

informal, British

Meaning

An older person, usually a woman, dressed in a style meant for someone much younger.

Where it comes from

A British expression from the 1800s, drawn from the butcher's trade, where tough old mutton was sometimes passed off as tender young lamb.

Why it is funny

The humor is the blunt, slightly cruel honesty of the comparison. It treats a person like meat at a market stall and quietly notes that the label does not match the cut.

Used in a sentence

"That sparkly mini-dress on her is a bit mutton dressed as lamb."