A Weekend at Bernie's is when you get your girlfriend so drunk that she passes out cold. Then you dress her up like a slut, prop her up doggy-style and do her in the ass.
"Dude. My girlfriend doesn't want to have anal sex with me, so last night I bought her 6 tequila shots, took her home, and when she passed out, I dressed her up like a hooker and fucked her in the ass! It was a total Weekend at Bernie's!"