You thawed some tuna 5 days ago is it still safe to cook and eat?

No, it is not safe to cook and eat.

According to the USDA, raw fish and seafood should not be stored in the refrigerator for more than 2 days before cooking or freezing. This includes thawed fish and seafood.

After 2 days, the fish will begin to spoil and develop harmful bacteria that can cause foodborne illness. Symptoms of foodborne illness can include nausea, vomiting, diarrhea, and abdominal pain. In some cases, foodborne illness can be serious and even life-threatening.

To reduce the risk of foodborne illness, it is important to follow recommended storage and handling guidelines for fish and seafood.