Green’s Dictionary of Slang

a person, usu. a woman, who dresses younger than her years.

2002

‘Tell Me About It’ Advice Column Wash. Post 28 Jan. 🌐 I remember vividly how hapless and lame older people used to seem to me when I was these kids’ age — now, though, I want to seem ‘with-it’ and relevant, without appearing to be MDL (mutton dressed as lamb) in spirit.