Bonce is a playful sort of a word. It's British slang for the sort of head that isn't too obviously overburdened with dignity, beauty or brains.
Still, nothing too bad can happen to your bonce. You can bang it, but not fracture it; it can be stupid, but not diseased; it can inconveniently block out the view, but not terrify the children.
In fact, if we all shed our dignity a bit and accepted that we have bonces then the world would be a jollier place all round.
Word To Use Today: bonce. In the 1800s a bonce was a type of ball, often made of terracotta, used for playing jacks.
The Children's Museum of Indianapolis https://commons.wikimedia.org/w/index.php?curid=21064230 (though that ball looks as it it's made of rubber, I'm afraid).
There's a theory is that bonce is a form of of the word bounce - though I wouldn't try bouncing anything made of terracotta, myself.