The term 'mutton dressed as lamb' was first used in the journal of social gossip that Mrs Frances Calvert compiled in 1811, and is possibly attributed to the then Prince of Wales (later George IV). At a musical evening, the Prince was asked whether he did not think some girl pretty. He reputedly answered, “Girl! Girls are not to my taste. I don't like lamb; but mutton dressed like lamb!”

In this case, ‘dressing’ refers to the preparation of food before cooking. The implication in 'dressed as lamb' is that the woman has prepared herself for a romantic encounter. 'Mutton dressed as lamb' was originally a disparaging description of a woman aiming to deceive men into believing she was younger than she really was; back then it was an economic necessity for a woman to marry while still of childbearing age.

Nowadays it refers exclusively to a woman who is deluded and thinks herself attractive in clothes, jewellery and make-up usually worn by those much younger. She’s probably looked in the mirror and thinks herself sex on legs, but she merely proves that eyesight deteriorates with age.

When you eat dirt for the first time and leave your family to build a shack in the woods

Middle aged women dressing/acting/pretending like they're much younger than they actually are.

Often abbreviated to "mutton".

When an older woman tries to dress young to be "cool". Usually involves jeans with rhinestones, Ed Hardy shirts, multi-highlighted hair, and some serious heels to finish off the look. Usually very embarrassing to her kids who are in either middle or high school. Still lives in the 80's with her music. Will still wait in line to see Bon Jovi.