A promise between 2 girls that cannot be broken. If a girl code is broken, it may end in a broken friendship. Girl code can only be used for important things such as crushes or secret make out seshes.
Brandon: Hey Julia who did Kelsy hookup with last week?
Julia: Sorry that's girl code! ;)
Jack: Who does Melanie have a crush on?
Donna: Sorry that's girl code! ;)