Wiktionary
hit the head
vb. (context US slang English) to urinate or defecate; to go to the bathroom
Usage examples of "hit the head".
As soon as he was near enough to hit the head of the animal, Edward levelled and fired, and Pablo let Smoker loose, who bounded forward over the hillock.
Her counter-spell hit the head of the handsome wizard, which was blown off and landed twenty feet away, gouging long scratches into the wooden floor.
She managed to get it around in time to hit the head of the polearm as it stabbed at Flora.
I've got to hit the head, he thought, and brought the ballista to bear.
However, although it did not hit the head, the projectile did bounce off the tip of the nearest of the webbed wings.