To fight violently is to fight "tooth and nail," with every "tooth" and every "nail." Example: "That woman fought tooth and nail during the war to protect her children." Animals sometimes fight violently with every tooth (in their mouth) and every nail (on their claws). Example: "That cat is terrified (very afraid) of cars; try to get him in a car and he'll fight you tooth and nail." To fight "tooth and nail" is to fight completely and with everything you have. Note: "Tooth" and "nail" are both singular in "tooth and nail." Example: "That was a tough match; they fought us tooth and nail!"