in. to seem suspicious. (See also fishy.) : Marlowe squinted a bit. Something smells fishy here, he thought.
Dictionary of American Slang and Colloquial Expressions by Richard A. Spears.Fourth Edition. Copyright 2007. Published by McGraw-Hill Education.
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Idioms & Phrases
Be suspect or suspicious, as in His explanation definitely smells fishy; my guess is that he's lying. This idiom alludes to the fact that fresh fish have no odor but stale or rotten ones do. [Early 1800s]