The Nine Weapons
There are two hands, two elbows, two knees, two legs, and the most important weapon is your head.
The Head as the Ninth Weapon
Head butt? Though ancient Muay Thai used head butts until changing the rules in King Rama 5 era (Including name changing to Muay Thai, and from Kard Chuek to boxing gloves) around the 1920s, BUT head butt is not exactly what ancient people meant for head to represent.