Slang term for a vagina that constricts or clamps tight when penetration is attempted. This can sometimes indicate a condition like vaginismus. More frequently, it's happening just because she has reconsidered and doesn't want to have sex with you now.
"Hey bro. Susan told me that last night after you guys went home you tried to finger her but couldn't even get that in there. What gives?"
"Oh, man... she's just got a case of cuntpucker, that's all."