When you are high and somebody or something is irritating you. Such as friends, parents, teachers, younger sibling, cops. Buzz Kills do things such as spit on people, beat on people for no reason, annoy you about something you have to do, etc.
Shit, we smoked a nice fat blunt and we walked home and about 3 minutes later the cops questioned us and by the time I got home I wasnt even high. Cops are such buzz kills!