Mutton dressed as lamb
An ageing woman who is dressed or made up as if much younger.
The term 'mutton dressed as lamb' is first found in print in the journal of social gossip that Mrs Frances Calvert compiled in 1811. Extracts from the journal were published in 1911 as An Irish Beauty of the Regency:
Some one the other day asked the Prince of Wales at the Ancient Music whether he did not think some girl pretty. 'Girl!' answered he, 'Girls are not to my taste. I don't like lamb; but mutton dressed like lamb!'.
It was slightly preceded by an earlier version, in which woman were said to be dressed 'lamb-fashion'. An example of that appeared in the anonymous 1810 novel Splendid Follies:
Ewe mutton without garnish is a tough bite, to be sure; but methinks she's dished herself off to day, lamb-fashion.
The 'dressing' of food was previously the term for the preparation of the item for cooking. The implication in 'dressed as lamb' is that the woman had 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 - it being an economic necessity for a woman to marry while still of childbearing age.
Its current usage, while still disparaging, is of a woman who is apparently deluded and thinks herself attractive in clothes usually worn by much those much younger - the motivation having changed from notions of marriageability to those of self-esteem.