Someone who's had a bad day, and uses their negative emotions against you, or someone in a terrible mood who tries to make everyone feel the same way. All in all, they bring everyone down with their negative energy to bring themselves up. Similar to a party pooper.
Mary D. and Elizabeth got so drunk at the party last night, they woke up in a pile of vomit next to an old man. When they went to school, they dumped on everyone they saw. Everyone did not enjoy shit on their faces, so they called Mary D. and Elizabeth "the day killers."