A secret code girls use to confuse their male friends or their boyfriends. This code when mentioned appears to be code for lesbian activity, but whether it is will never be known to men. If you're a girl, you know what it means, but if you're a guy, you'll never know.
Amie: let's code 453,Judie, these boys are lame.
Rickie & Chris: what is that????
Judie: you'll never know.