the apparent image of a fishwife is one of a grouchy, rude, loud mouthed woman. So I thought about being this woman. Smelling of fish 24 hours a day, every day of my life would make me grouchy. Sleeping with a man who smelled like of fish 24/7 would make me even grouchier. Watching people come to me to buy my fish, and then stick up their noses at the smell, would make me rude. Because I am not the only fishwife selling my husband's catch, I know the one with the loudest voice will get the customers. Perhaps this is how the image became set.
A fishwife is a woman who sells fish. She's not necessarily married to a fisherman. Wife here is from the Old English wif meaning woman. It's likely that fishwives got their reputation from having to cry their wares in the London fish market at Billingsgate, which itself has become a byword for profane verbal abuse.