My Gran was sometimes a bit of a curtain-twitcher, if truth be told, keeping a close eye on the neighbours and their activities.
One lady was frequently the object of her attention. When the said lady was going out somewhere special and wearing her best clothes, Gran would mutter about 'mutton dressed up as lamb'.
The meaning of the phrase is quite clear - that she was trying to appear younger than she actually was.
Originally it was not derogatory. The term is first found in print in 1811 in the journal of social gossip by Mrs Frances Calvert.
"Someone 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!'."
Nowadays though, it suggests that a woman is deluded and thinks herself attractive in clothes usually worn by much those much younger.
Of course, it doesn't need to apply exclusively to females. When I don my red trousers, perhaps I'm ram dressed up as lamb.