Its when you place a hard penis on someones forehead. The balls will also be resting on their head. It resembles a unicorn. Placing a soft penis on someones head is baby unicorning. And if this takes place in a pool or water it is known as narwhaling.
During a party you find a passed out person and place your hard dick on their forehead, balls at the base of the forehead, this is known as unicorning.