fluke

Old English flōc meant a flounder or flatfish — the flat thing in the water. The word shares its root with Old High German flah, 'flat,' and is related to the modern German flach. The flatness was the defining quality: the fish distinguished by lying horizontal, by being two-dimensional against the seafloor.

The same word named the flat triangular tip of an anchor's arm — the part that catches in the seabed — and the two lobes of a whale's tail, both flat and triangular. In the 16th century, a fluke was any flat, triangular blade: the head of an arrow, the arm of an anchor. Sailors knew all these senses.

The jump to 'lucky accident' came from billiards in the 1850s. A fluke shot was a ball that went in by accident — the ball deflecting unexpectedly off something, the result good but not intended. The billiards term may have come from the anchor fluke, which catches randomly in the seabed — finding purchase not by aim but by drift. By 1900, fluke meant any unintended success.

The parasite that infects liver tissue is also called a fluke — named for its flat, leaf-like body, back to the original Old English sense. So in English one word covers flatfish, anchor-hooks, whale-tails, parasites, and lucky accidents. The unifying idea is always flatness — except in the lucky-accident sense, where the image is of something catching by chance, like an anchor finding unexpected hold.