The word "咬牙切齿" is formed by combining two parallel verb-object phrases: "咬牙" and "切齿". "咬" means to bite, "牙" means teeth, together meaning to grind teeth; "切" here means to gnash or clench, and "齿" means teeth, together meaning to clench teeth. The combination reinforces a single vivid action of teeth grinding to depict intense anger or resentment.