To shift someone means to snog them, proper teenage-style, usually on a night out or at a house party. It’s mainly kissing and making out, not the full hook up job. You’ll hear it in schools, colleges, and among adults who never really grew up. If someone asks who you shifted, they want the gossip.
"Did you shift anyone at the disco last night? I shifted yer man in the GAA jersey behind the smoking area. I was only a small bit locked."