This literally means "fishmouth". It was originally a derogatory slang word for a dockside prostitute. As you might imagine, there are many popular explanations for the expression, each more obscene than the last. It has spread far beyond the docks, even into the mountains and onto the mainland, as a very strong, general purpose insult.
If you're sitting in a Nevashi bar, wondering what it might be like to be hit over the head with a bottle in a brawl, try this sentence:
Mi an amá dei go shufuma. ("Your mother is a fishmouth.")
On the other hand, uma shufi just means "mouth of a fish". If you call someone (or their mother) that, they'll know what you intended to say, but the effect will be more comedy than insult. They might hurt themselves laughing at you, though, so you might get your revenge after all.