Stories We Like: A Guide to the Comma
late 15c., from fish (n.) + -y (2). Sense of "shady, questionable" is first recorded 1840, perhaps from the notion of "slipperiness," or of giving off a bad odor.
Very probably false or dishonest; very dubious: The whole proposition is decidedly fishy
[1840+; probably fr the unpleasant odor of spoiled fish]