fluke (n.1) Look up fluke at Dictionary.com
"flat end of an arm of an anchor," 1560s, perhaps from fluke (n.3) on resemblance of shape, or from Low German flügel "wing." Meaning "whale's tail" (in plural, flukes) is 1725.
fluke (n.2) Look up fluke at Dictionary.com
"lucky stroke, chance hit," 1857, originally a lucky shot at billiards, of uncertain origin.
fluke (n.3) Look up fluke at Dictionary.com
"flatfish," Old English floc "flatfish," related to Old Norse floke "flatfish," flak "disk, floe" (see flake (n.)). The parasite worm (1660s) so called from resemblance of shape.