Fluke

A fluke in the sense of a piece of accidental luck or good fortune dates from the mid-19th century when it was specifically a colloquial expression for a lucky or unplanned shot in billiards. This usage has since been expanded to cover any lucky or fortuitous result. No one knows for sure just how fluke came to acquire this meaning. Some sources insist it derives from the spear-shaped ends of a ship's anchor, which are also known as flukes, but there is nothing particularly 'flukey' about anchoring a ship. Anchors either grip or they don't, and it's good seamanship rather than a fluke when they do. The flukes on a ship's anchor are also similar to the shape of a whale's tail, and hence fluke is also used to describe this part of a whale's anatomy, but the connection or relevance here to a stroke of luck or good fortune is obtuse to say the least. There is, however, one more type of fluke, which means a flat fish or flounder, and the OED offers the thought that fluke might be a pun on flounder, where people who flounder often rely on a fluke or stroke of good luck to rescue them from adverse circumstances. This seems to be a bit of a stretch on the OED’s part, so it is safer to stay with the billiards story where fluke shots are a common occurrence.