An attractive female who studies maths or has a degree in it.
However it can generally be used to describe any attractive and intelligent girl.
"Brooo, you see that fit girl walk out of science class?" Damn she's a proper countdown girl"
Countdown Girl - A female that looks good from a distance, but her appearance becomes progressively worse upon approach.
"I thought she was hot, but she ended up being a Countdown Girl."
"What do you mean a countdown girl?"
"I thought she was a 10, but as I was walking up to her she turned into a 9...8...7...6..."
A person that every time you see them their Hotness level drops, beer or any other such judgment imparting drink or substances not involved
Dude, she is a countdown girl, yesterday she was a 10 and today shes a 9, who knows how far she will fall.