Put a damper on

Meaning

Make dishearted, especially to diminish interest in something that was previously exciting.

Origin

'Putting a damper on' something would seem to have a clear relation with damping down a fire, i.e. putting water on it to dowse it. A damper isn't a material object but just 'something that depresses the spirits. The word is used in that way in Samuel Richardson's Clarissa; or the history of a young lady, 1747–48

"I very early discharged shame, that cold water damper to an enterprising spirit."

It was also the name of a snack taken to diminish the appetite, as in Maria Edgewiorth's Popular Tales, 1804:

"In the kitchen, taking his snack by way of a damper."

The figurative use of the phrase doesn't appear in print until the mid-19th century. This is from J. F. Murray's World of London, 1843:

"If the clerk of the weather office is determined to put a damper on the festivities."

See also, put the mockers on.