A person in a large group, such as high school or summer camp for example, who is so unpopular and unnoticed and does so little that many people do not even know who he/she is. Originally short form of tertiary, where primary is used to describe the most popular or well-known in the group, and secondary people are average and known by a few people in the group.
person 1: "hey, you know that guy Jonny York? He's such a tersh"
person 2: "Who the hell is Jonny York?"
person 1: "Exactly!"