To nick in behind everyone’s back

[Chicken Sandwich]

Gareth: What do you think of her?
Tim: A bit out of your league, mate.
Ricky: Definitely out of your league.
Gareth: No. Smug. You think you're so... just 'cause you got a bit of Donna because, I played by the rules 'cause she was out of bounds. You nicked in behind everyone’s back. I'd have got a bit if I'd known it was up for grabs. I'd have done her, but that would be shitting on your own doorstep.And anyway, I don't do sloppy seconds.

This is a bit of dialogue from The Office (UK). "To nick" can mean "to steal", but why does he say "to nick in"? What does this mean?

Thanks.

[iKitty]

NOT A TEACHER

Are you sure it's nick? Usually the phrase is nip in behind [someone's] back. Nip means to move quickly in this context.

[JohnParis]

I think the author means "snuck" (the past tense of sneak) more than he means to use the literal meaning of nick (to steal).

John

[Chicken Sandwich]

NOT A TEACHER

Are you sure it's nick? Usually the phrase is nip in behind [someone's] back. Nip means to move quickly in this context.

You're right, it's "nip" not "nick". The English subtitles, from which I copied the phrase, were wrong.