Taking the wind out of your sails...
A sailing ship passing to windward of another will blanket the wind blowing on its sails and as a result the ship will slow down, or in some cases it might even stop! This was a tactic that was originally used in warfare and later by pirate ships, but it is still very common in yacht racing today.
The phrase now extends beyond sailing as we use it to describe someone or something that deflates us – causing us to slow down, or stop.