I have looked at the meaning of fishwife at Collins Language (I can't link directly to the definition) and it tells me:
fishwife n (pl -wives) a coarse or bad-tempered woman with a loud voice
Other places I've looked say similar things, with the additional definition of
a woman who sells fish.
Which I'm guessing is the original meaning.
But I can't make the jump from someone who sells fish to someone who is mean, bad tempered, speaks bad language and is loud.
Looking at Etymonline only yields
from fish + wife in the woman sense.
Which is nice, but not helpful here.
Why are fishwives mean?
Best Answer
The definition of a fishwife is tied to both of the definitions you listed. Wikipedia's article on the fishwife explains that:
Unlike its common usage today, wife meant any woman, not just a married one. Some words still retain this meaning:
So historically, a fishwife was just a woman who sold fish. Over time, since fishwives were often "loud and foul-mouthed," their job title became synonymous with your definition of "a bad-tempered woman with a loud voice."
Interestingly, fishwives have had different reputations in different areas. In Billingsgate, there were "the wives of Billingsgate" who:
On the other hand, the fishwives of Newhaven, Scotland were: