New version of the old saying "Mutton dressed as lamb", and is a play on the old saying "Wolf in sheep's clothing."
Gives reference to the woman being a cougar while dressing as a skanky teenager to appear less predatorial and more attractive to her unsuspecting prey; hot young males.
This disguise can sometimes be successful at getting them closer to their game, or they can just look terribly tragic.
Dave "Whoa, look at her..."
Andy "Mate, stay away, she's a Milf in sheep's clothing."