Term used to describe the angry-looking facial expression that one instinctively makes when s/he is resting, zoned out, or not actively engaged in a conversation. Someone with an angry resting face may be unfairly perceived as cold or aloof. An effective remedy for an ARF is to learn to "smile with your eyes."
"That Alison looks like such a bitch."
"Nah dude, she just has an ARF (Angry Resting Face). Girl's gotta work on that."