Figuratively, a “wet blanket” is a person or object that destroys other people’s enjoyment.
In other words, a party-pooper, a spoilsport, a killjoy, or Joe Btfsplk, the most unlucky denizen of Dogpatch, featured in the newspaper comic strip Lil’ Abner (1934 – 1977). Mr. Btfsplk was such a killjoy that he travelled with his own black cloud overhead. Add to this list Eeyore, the always-pessimistic donkey from A.A. Milne’s Winnie-the-Pooh stories.
The expression goes back at least to the 1660s, according to the Online Etymology Dictionary, when it referred to a simple way to fight small fires. One used a thoroughly wetted blanket to smother a sudden flare-up, especially in confined spaces.
More than 200 years later, the term appears in an article titled “Wet Blankets,” in Godey’s Lady’s Book and Magazine, February 1871, and here it is clearly used in the figurative sense we’re familiar with today.
The phrase is, however, still being used in the original sense. “Weapons emplacements should use a wet blanket canvas, or cloth to keep dust from rising when the weapon is fired.” From Combined Arms Operations In Urban Terrain, Department Of The Army, Washington, DC, 28 February 2002.
I wonder if a wet blanket would work on a two-year-old having a tantrum.