1911 Encyclopædia Britannica/Fluke

FLUKE (probably connected with the Ger. flach, flat), a name given to several kinds of fish, flat in shape, especially to the common flounder; also the name of a trematoid worm, resembling a flounder in shape, which as a parasite infects the liver and neighbouring organs of certain animals, especially sheep, and causes liver-rot. The most common is the Fasciola hepatica (see Trematodes). It is also the name of a species of kidney potato. Probably from a resemblance to the shape of the fish, “fluke” is the name given to the holding-plates, triangular in shape, at the end of the arms of an anchor, and to the triangular extremities of the tail of a whale. The use of the word as a slang expression for a lucky accident appears to have been first applied in billiards to an unintentional scoring shot.