"Just A Slit" refers to the occurrence of when you end up seeing an extremely hot woman naked and her vagina has no detail what-so-ever. It just looks like an open wound; a slit. This phrase is (ONLY) used with a model caliber woman, because the expectation of her entire body is very high. It would be just like a woman seeing a model caliber man with a tiny, ugly penis. It is most commonly used when referring to a model-type woman who is very conceded. A typical person pretty much always wants someone who is conceded to have a flaw that they can't do anything about. The Karma-Factor.
(Man 1) SH*T man, I got lucky enough to spend a night with this hot ass chick I met at the club last night. She pealed off her cloths like wrapping paper. Nice breasts, nice ass, nice legs; but then her pussy was Just A Slit... I still F***ED her of course! But man I was expecting a more beautiful pussy from a model like that... (Man 2) Are you going to call her? (Man 1) F**K no! She was conceded as H@LL!!! You know conceded chicks are worth the trouble for dating.