Any experience that feels like having a bucket of cold water thrown over your head. Something that produces a corresponding comedown in mood in the person experiencing it.
"Jeez, when Ifound out the price of the drinks, it put a right old dampener on the proceedings."
A dampner is a hydraulic device used to reduce pulsations in a hydraulic system.
This differs from a damper which is a hydraulic device used to damp vibrations in a physical system that the hydraulics are connected to.
A Pulsation Dampner is used in a piping system, generally adjacent to the source of the flow or pressure disturbance, which is typically a modulating element like a pump or a flow control valve.