What would you say to your boss if he is crazy about an idea, but you think it stinks?

I will try to point our various ways that I believe we may approach the matter. But if he keep on insisting I will personally implement his Idea and try as much as possible to minimize negative impact to the organization.

Sir, I learned many principles from you as you are the most qualified for over years in long professional run. I'm also so honoured to share my professional views with you as I my ideas can be reviewed by you and if hits up to the benchmark then it would be my best performance leaded by your orders as my boss and I would be grateful to you if I can win your appreciation as an ultimate decision maker of the company.

If the idea really stinks and boss sticks on it then I will definitely abide to his ideas since he is the ultimate boss and I'm also responsible to keep my position safe. I don't want to miss the opportunity given.

I would talk to my boss and will tell my logics and doubts about that idea and will ask from him for some assistance and perspective upon that. If he is not satisfied with my ideas then I will try to settle down with his version of plan as he is the boss and have gained experience in this field for a longer time.