1
general::
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]
American Heritage Idioms
2
general::
Phrase(s): smell fishy
to seem suspicious. • Barlowe squinted a bit. Something smells fishy here, he thought. • Something about the deal smelled fishy—too good to be true.
McGrawhill's American Idioms And Phrasal Verbs