While riding your scooter, you bang your forehead on the car in front. Your scooter's wrecked, so you have to carry it home. You'll need a lot of luck to get that scooter home safely.
As you carry the scooter, your luck turns, and it starts to rain, so you pull out your umbrella (うん). Now you're holding your scooter, your forehead, and an umbrella.