Fishy means smelling or tasting strongly of fish. A fishy smell, fishy water, it tastes fishy.
Fresh fish should not smell fishy — the smell is a product of decay, so fishy is a complaint rather than a description. The figurative sense is much the commoner: something fishy means something suspicious, there is something fishy about it. That use probably comes from the idea of a fish being slippery and hard to hold. Note fish for compliments; and a fishy look, meaning cold and staring.
1adjectivefishySmelling or tasting strongly of fish.
“The cat's breath smelled fishy after its dinner of sardines.”
2adjectivesuspiciousSuspicious.
“There is something fishy about the whole arrangement.”
fishy is a quality of fish smell or suspicion.
The word is fish plus -y. The suspicious sense appeared in the nineteenth century, probably from the slipperiness of a fish rather than from the smell, though both explanations have been offered.
Middle English
fishi
English, today
fishy
From Middle English fishi, fischey, equivalent to fish + -y.
Etymology from Wiktionary, CC BY-SA