An expression requesting a picture of naked breasts to serve as proof of having a girlfriend. Also used to question an online poster's sexual identity.
Rosencrantz: My girlfriend gave me such a great blowjob
Guildenstern: Girlfriend? Hah! Tits or it didn't happen!
Philomena: I'm a gorgeous brunette with large breasts looking for World of Warcraft fans so I can suck their cocks
Nerdcore: No you're not, you're a guy.
Philomena: No, I'm a girl
Nerdcore: Tits or it didn't happen.