Dirty pool noun phrase informal
Behavior that is not right according to a set of rules or principles or intends to make someone believe in untrue things.
A: His friends ordered everything on the menu and left to make him pay for all that thing. B: What dirty pool!
The retailer was apprehended due to his dirty pool. Before being arrested, he had tricked numberless people.
This phrase dates back as far as 1918, and may go back a bit farther.