fluke 1 
(flk)
(flk)n.
1. Any of numerous parasitic flatworms, including the trematodes, some of which infect humans, and the monogeneans, which are chiefly ectoparasites of fish.
2. Any of various flatfishes chiefly of the genus Paralichthys, especially the summer flounder.
[Middle English, flounder, flatfish, from Old English flōc; see plāk-1 in the Appendix of Indo-European roots. Sense 1, from the flounderlike shape of sheep flukes .]