[descriptive noun]
Describes a person who essentially ruins other peoples fun, either because they openly disapprove of what other people are doing, or because their unpleasant attitude annoys or depresses everyone who would otherwise be enjoying themselves, if it werent for that person acting like a wet blanket.
The term is probably American in origin, first being used as slang in the mid 1800s. It was meant to compare a person to a wet blanket used to extinguish fires by smothering them, similar to how such a person would extinguish fun and excitement in social situations. Another, more modern interpretation of the term likens a dislikable or overbearing person to the aggravating, uncomfortable feeling of being covered by a blanket when its wet.