<interjection> Chiefly British;
1) Expression of victory or happiness of the result of a given situation.
Get in (there)
2) Phrase used to encourage another to do something dangerous or exciting, usually when they show signs of wanting to do it anyway.
CARL: "Here, Pete! Jenny says she wants to give you a hand-job."
PETE: "Get in! Where is she?"
JENNY: "You like that?"
PETE: "Oh yeah, harder bitch! HARDER!"
<enter Pete's friends>
PETE'S FRIEND: "Haha, whoa! Get in there, lad!"
PETE'S FRIEND 2: "Ogh, that's fucking sick..."