What is 1661?
refers to an older woman in 'younger' clothing; looks 16 from the back, but 61 from the front.
Ew, how 1661 is she?
A bird that looks young and fit from behind, but when you see the front she is old and ugly.
Driver: Look at the fit arse on that bird.
Passenger (as passes in front): Unlucky mate, she's a 1661.