cop off with (one)

1. slang To kiss passionately and at length with one, especially during a chance social encounter. Primarily heard in UK. I saw Mike copping off with some random girl in a corner of the bar, so I took a picture and sent it to his girlfriend. I definitely didn't expect to see my year four crush all grown up at the bar—or to cop off with him once we were both a little tipsy! I have to tell you something—I copped off with Josh tonight! And I have to tell you something else—he's a really good kisser!

2. vulgar slang To have sex with one. Primarily heard in UK. I wouldn't cop off with your roommate if I were you. I heard that Katie copped off with Brad last night after they left the bar together. You're still dating Jenny, right? Have you copped off with her yet?