A person or thing that for some reason (e.g. for being annoying, a whinger, a loser, or boring, etc.) drains one's energy and general enjoyment of life.
"Did you go to Stacy's party Saturday night?"
"Yeah, total drainer."
"DRAAAAAAAAAAAINER" (Slow chant to remove offending drainer from giving a speech on some kind of public pedestal)
Greg - "Yes mate, done her"
Dave - "You gonna make effort with her and start a reationship?"
Greg - "No way pal, Shes just a drainer !"