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]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.