In law, a fishing expedition is an attempt to gather evidence with the use of tactics like vague questions for someone on the witness stand or in a deposition, or broad legal requests made during discovery to try and gain access to compromising materials. People can also use this term to describe situations where a lawyer threatens suit or other legal action with intimidation in mind, not necessarily with the law on his side.
Ethically, fishing expeditions are dubious in nature. In a court of law, an attorney can object to questions too vague or broad on the grounds that they are not directly relevant to the case and might lead a witness into making a damaging statement. The judge can sustain it if the attempt clearly appears to be be fishing, or may overrule the objection and allow questioning to continue, usually with a warning to make sure the attorney gets to the point quickly.
In the process of discovery, where people on both sides of a case must share certain kinds of evidence and can submit questions and orders to turn over specific materials, a fishing expedition can be a common tactic. People may not have enough information to feel confident pursuing the matter in court, but might be able to turn something up with a little fishing. They can word questions vaguely in the hopes of tricking someone into making an admission that would help their case, or can broadly interpret orders for documents and other materials to get as much information as possible.
People often use this term in a derogatory way when someone appears to be grasping to pull a case together. The person conducting the fishing expedition usually wants to do so subtly to avoid alerting people to the fact that she doesn't have a strong case or is afraid of information that might turn up in court. In some cases, the other side may use the evidence of a weak case to push for a plea bargain or settlement.
If people suspect a fishing expedition is stepping beyond the scope of the law, they can request intervention from a judge. The judge can review the behavior in question and decide to allow it to go forward, or order the person to stop and issue a warning against future episodes of similar behavior.