miller's-thumb definition by Webster's New World
Webster's New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio. Used by arrangement with John Wiley & Sons, Inc.
sculpin (sense )
miller's-thumb definition by American Heritage Dictionary
Any of several small freshwater sculpins of the genus Cottus, especially C. gobio, found in Europe and North America and having a large spiny head and spiny fins.
Origin: 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).