This word was commonly used in the British magazine PC Gamer and later spread from their official forums to several IRC channels and other forums.
When someone would make a joke that is funny as well as slighting shocking (although this last part has become optional) people would reply with a simple "Arf!"
By Jesse
Arf
Term meaning a person of African decent who cannot swim or swim well. Derived from the acronym A.R.F. which stands for African Rock Fish. Used by lifeguards or ride attendants in the U.S. northeast who generally work at the jersey shore or water parks near large urban areas.
ass. rack. face. Guys say arf if there is a good looking girl within hearing distance. arf stands for ass rack face. If a girl is an arf, it means shes hot.