For the ever loving fucking fuck fuckity fuck it is CANON.
can·on [kan-uhn], noun
- an ecclesiastical rule or law enacted by a council or other competent authority and, in the Roman Catholic Church, approved by the pope.
- the body of ecclesiastical law.
- the body of rules, principles, or standards accepted as axiomatic and universally binding in a field of study or art: the neoclassical canon.
- a fundamental principle or general rule: the canons of good behaviour.
- a standard; criterion: the canons of taste.
can·non [kan-uhn], noun
- a mounted gun for firing heavy projectiles; a gun, howitzer, or mortar.
- British Machinery, quill ( def 10 ).
- Armour. a cylindrical or semi-cylindrical piece of plate armour for the upper arm or forearm; a vambrace or rerebrace.
- Also called cannon bit, canon bit. a round bit for a horse.
- the part of a bit that is in the horse’s mouth.
As can be clearly seen and understood from the above, HEAD CANNON would be a significantly different thing from what you mean, and could also result in serious bodily harm (to you or your loved one to whom you are giving head).
This PSA has been made in the interest of keeping my head from exploding.
Like a cannon.