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Any of several small freshwater sculpins of the genus Cottus, having a large spiny head and spiny fins, especially the European species C. gobio.
[From its stocky, thumblike shape (the phrase miller's thumb was originally a folk expression referring to millers who gave short weight by tipping the scales with their thumbs).]
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
(Animals) any of several small freshwater European fishes of the genus Cottus, esp C. gobio, having a flattened body: family Cottidae (bullheads, etc)
[C15: from the alleged likeness of the fish's head to a thumb]
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014