Plural of blueside; a type of fish (bluegill or blue-sided fish) or a poetic/regional term for certain blue-colored aquatic creatures.
Compound of 'blue' + 'sides' (referring to the blue coloring on the sides of certain fish), likely a regional or colloquial term that emerged from direct observation of fish coloration.
This is a great example of folk taxonomy—fishermen and local people create their own simple names for fish based on what they observe (blue sides = this fish), and sometimes these names stick regionally even when they're not in formal ichthyology.
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