Perhaps a compound of “finn” (white, bright) + “cosc” (control, restraint, hindrance, punishment)? What is this chief called? Is it a character from Irish mythology? It would be helpful if you could give us some background information.
Sounds intriguing. 🙂 Another wild guess: fionnuisge or fionnuisce means “freshwater”, though I’m not sure making freshwater would be considered much of a feat… Hopefully someone else has an idea.