A girl next door is basically one of two people. She's either the girl you first had sex with and didn't dare tell anybody about it -- because she was less than a 5. OR, she was the girl you wish you had sex with and told everyone you did anyway -- because she was such a hottie.
Friend 1: Did you check out that new girl next door?
Friend 2: Yeah man, I did her yesterday.
Friend 1: No way, you didn't do her...
Friend 2: Sure I did... right after doing your sister.