An investigation with no defined purpose, often by one party seeking damaging information about another. Such inquiries are likened to fishing because they pull up whatever they happen to catch.

Legal grasping at straws; the use of pre-trial investigation (discovery) or witness questioning in an unfocused attempt to uncover damaging evidence you can use against your adversary.

Fishing expedition is a colloquial, informal legal term often encountered in the U.S. and other similar legal systems (and especially prevalent in TV and movie dramas). It is normally used by the defense to cynically refer to the prosecution's attempt to undertake more intrusive searches of a defendant's premises, person, or possessions when (in the defense's view) there is insufficient probable cause to carry out such a search. The metaphor likens this to a fishing trip on which one has insufficent information about conditions at the fishing spot, and may or may not catch any fish.