A girl who looks good sitting down but when she stands up it is clear that she has a large bottom or thighs and is therefore much less attractive than anticipated.
A girl or group of girls (sitters) arrive early at a pub or bar and get a good seat. They remain seated all night so that nobody sees their unattractive half. Then, you spent some time chatting her up and when she gets up to go home with you/give you a kiss whatever, you see that she is much less attractive than anticipated. On a night out you need to watch out for sitters- they deliberately try to trick you!!