Stands for Acute Rocket Syndrome. As highly good looking girls are called "rockets", a girl with ARS is extremely good looking.
A rocket usually begins at 9.0 on the traditional 10 point attractiveness rating scale and thus a girl with ARS represents a girl above a 9.0 (9.1+).
"Starbucks was filled with girls who had disorders last night."
"Disorders? What kind of disorders?"
"ARS buddy, it was great."
"Ohhh, ARS. I should have come and met you for a coffee."