What is the origin of the phrase "there's something fishy"?

The meaning of 'there's something fishy' is something of doubtful character or of some shady deal where the motives are suspected. The term 'fishy' is thought to have arisen from the notion "as slippery as a fish" or as an allusion to meat with a "fishy" taste, an indication that the meat is bad.

A simpler explanation is that old fish (not good to eat) smells fishy with bad odour caused by a chemical named trimethylamine which is released when fish begins to break down.