When you take the wind out of someone’s sails, you succeed in denting his confidence. You achieve this by doing or saying something that the individual does not expect.
* Usha was really angry with her husband. But when he came home with flowers and a saree, it took the wind out of her sails.
You have to remember that in the old days, ships depended on the wind to get from one place to another. If you were to rob a ship that is sailing next to you of the wind, what would happen? The wind would be taken out of its sails and as a result, the ship would stall.