When something is fucking sweet. If you meet someone who you really like, they're in-shop. When someone is a douche, they are definitely not in-shop, or rather, "out of shop".
Furthermore, if a girl wants you, you can describe it as..."yeah I gave her a delivery, she wants my gargantuan."
"Met a girl at a party tonight, she was in-shop"
"This guy tried to come into JJ's at 330, he was not in-shop"