The origin of wet blanket is very literal. Way back in the day (the 1600's day) it was recommended that people keep a small wet blanket near their fires in case they got out of control. Then they would have a quick way to put the fire out before it got too big or out of control. ( source ) Throughout the many years the phrase morphed from literal to figurative and it showed up in 1871 as the way we use it today - someone who spoils other people's fun by failing to join in with or by disapproving of their activities.