For every party, there can be an equal and opposite party pooper. For every ointment, there is often a fly. Such a killjoy or spoiler is known in popular slang as a buzzkill or sometimes a buzz kill. The arrival of local law enforcement could be seen as a buzzkill for a party, or the raising of bright house lights could be a buzzkill for concertgoers. Anytime a harsh reality interrupts a person's euphoria or "buzz," it could be seen as a complete and total buzzkill.
There are few hard and fast rules concerning buzzkills. Some people realize they are deliberately coming across as killjoys to others, as in the case of parents breaking up a teenager's illicit party. Others may not intend to ruin another person's buzz or altered state of reality, but they have obligations of their own to fulfill, much like a bartender asking patrons to leave the premises at closing time. While the bartender may be seen as a buzzkill, nothing lasts forever and a certain jolt of reality could be necessary.
The slang use of the word buzz to describe a natural or chemically induced euphoria can be traced back several decades, but the derogatory term buzzkill appears to have gained popularity during the 1990s. The standard admonition at parties and other events was never to be a buzzkill for others. Sometimes circumstances such as curfews or neighborhood complaints could act as natural buzzkills for parties, but individual partygoers were strongly urged to go with the flow and not become buzzkills or killjoys for others.
Sometimes the terms being a buzzkill and harshing a buzz are used interchangeably. When a person has reached a certain level of euphoria, especially as a result of alcohol or marijuana consumption, he or she tends to want to hold onto that feeling as long as possible. Any interruption of that altered state of reality is not going to be received very well, and the interrupter may very well earn the dubious title of buzzkill. Sometimes circumstances do override personal indulgences, however, and there are worse names to be called than a buzzkill.