Fishy

Smelling like fish, or seeming suspicious and not trustworthy.

From 'fish' plus the suffix '-y' (meaning having the quality of). The literal meaning (smelling of fish) dates back centuries. The figurative 'suspicious' meaning emerged in the 19th century, possibly because fish markets were associated with deception and bad odors, or because something 'fishy' seemed slippery and hard to grasp.