if a situation or an explanation smells fishy, it causes you to think that someone is being dishonest Webbers's account of what he was doing that evening smells a bit fishy to me.
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]