The phrase 'Fishy Smell' or “Something Smells Fishy” is used when something seems suspicious.
Example of Use: “When I heard his account of the car crash, I noticed a bit of a fishy smell; he was obviously not telling the truth.”
The origin of the term 'fishy smell' alludes to the fact that while fresh fish have no noticeable odor, rotten or stale ones really stink. It first came into use in the early 1800s.