What is the origin of the idiom "fishing expedition" and when to use it?

The idiom "fishing expedition" is derived from the literal act of fishing, where individuals cast their lines into the water with the hope of catching fish. In this context, a "fishing expedition" refers to an exploratory or speculative venture in which someone is searching for something without a clear plan or specific target. The term has been used figuratively since at least the mid-20th century. This expression is often used to describe an investigation, inquiry, or search that lacks a clear purpose or reasonable cause. It implies that the effort is akin to casting a wide net or conducting a search without knowing precisely what one is looking for, and it may be seen as speculative or exploratory. In legal contexts, it can refer to a request for information or documents that appears overly broad or made with the intent to harass rather than to obtain specific relevant information.