miller's thumb

mill·er's thumb (mlrz)

n.

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.

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[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).]

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miller's thumb

Noun

a small freshwater European fish with a flattened body [from the alleged likeness of the fish's head to a thumb]