Will means to want or to choose. If you have free will, you are allowed to choose what you want. If you make a will, you've write down what you want to have happen to your money and your stuff.
You may have heard the expression, "Where there's a will, there's a way." It means that if you want something enough, you'll find a way to make it happen. If you impose your will on someone else, you're making them do what you want, as opposed to what they want. If you are willful, you are impulsive and always do what you want. If you will something to happen, you try to make it happen just by the force of your wanting it to––you might will a car to keep running, even as the gas needle sits at empty.