Its when you look at someone in the eyes, and you can just tell that there is nothing there, or they just look right through you, like you're prey. Sociopaths are said to show this trait, as well as some mentally handicapped. Check out photos of charlie manson, john wayne gacey, any of those serial killers.
They say the eyes are a window into the soul , someone with shark eyes is someone who has that vacant glare of danger in there eyes , where there not totally all there and there emotions and anger appear in there eyes , people with shark eyes are best avoided .. Ive seen many people in my life with that look in there eyes and there left alone.
Mike Tyson had shark eyes ... you can see the violence in his eyes .. people seen as violent or crazy tend to have shark eyes