I wanna get lost with you means that you and your dream lover want to sneak off together somewhere, where nobody will (hopefully) find you and, literally, for lack of better term, screw day and night.
I wanna get lost with you. Let’s go to the Northernaire Motel. Nobody will find us there. Or, even want to look there. Ahhh! Ecstasy! Bliss!
A quote from Guns n' Roses. Fom their song "You could be mine" on the "Use Your Illusion II" album. Also found in the liner notes of their first album "Appetite for Destruction" (at least it is on the vinyl version, dunno about the CD).
Its basically the kind of thing you say to a chick or a guy who likes you, but you dont wanna know cause they like rap music and you like rock.