Noyer le poisson, literally “to drown the fish”, means to confuse someone during a conversation or argument. The closest English idiom is “to cloud the issue”. The expression dates back to the 20th century. Obviously, it seems impossible to drown a fish. Well, the expression is a reference to a fishing technique: the fisherman would exhaust the fish, getting it out of the water and putting it back in. Once he would not feel any more resistance from the fish, then he would catch it easily!