1
A griefer is a smelly, brainless idiot who messes with your game just to make you lose your cool and quit the group for no damn reason.
You keep dying in the same spot, and it's because some griefer won't stop spamming you with arrows.
You log off, and they keep killing you just for fun.
You finally leave the clan, and they still send you a message saying 'I'll haunt you in the next game.'