illegal business; backdoor dealings
Refers to business conducted through unofficial, often illegal channels, such as unlicensed operations or under-the-table transactions.
That shop was busted for illegal business operations.
闇営業 emphasizes underground or clandestine operations, often with a stronger connotation of secrecy and illegality, while 裏口営業 focuses on the backdoor or unauthorized nature of the business.
Compound of 裏口 (back door) and 営業 (business), literally 'backdoor business'.