Slash -- Yes, I Once Drugged My Girlfriend's Mom ... So We Could Bang on the Couch

Slash was once so desperate to get laid ... he and a girlfriend drugged her mother, and then had sex right next to her while she slept -- and best of all ... he was 13.

The guitar god made the confession when we asked him about the craziest place he's ever banged ... fully expecting some story about life on the road during the Guns n' Roses heyday.

We definitely were NOT expecting this pubescent/criminal tale.

You've gotta hear Slash explain the whole caper, but the take away? In his words, "There's no ridiculous place [to bone] ... if you can get away with it."