A term used in gaming, most likely stemming from MMORPG lingo, where shortening terms becomes essential for text communication. A debuff can be used in two similar respects. It replaces the drawn-out phrase "negative status ailment", where an attack or spell causes some ill effect on it's target, i.e. poisoning, silence, bleed. Debuff can also be used referring to an attack or spell that removes positive status effects. Many traditional RPGs have adopted this term as well.
The noob stood no chance against the terrifying onslaught of poison and slow debuffs, and was soon witnessed yelling "wee wee wee," all the way home.