underground bank; clandestine bank
Refers to an illegal, unlicensed financial operation that provides banking services outside regulatory oversight, often associated with loan sharks or money laundering.
The police are cracking down on underground banks.
Using an underground bank carries the risk of being forced into high-interest loans.
Compound of 地下 (underground, clandestine) and 銀行 (bank). The term is a straightforward calque of the English 'underground bank'.