A code phrase for sex. This is derived from the emoticon codes in MSN Messenger, which to get a row of emoticons for a moon, an envelope (or, letter) and a woman/girl you type the code (S), (e) and (x). Sometimes moon letter girl is further disguised as the acronym 'MLG'.
Darcy: What do you wanna do tonight?
Tom: I dunno... Moon letter girl comes to mind.
Darcy: My house at eight?