Tard = mentally handicapped or just plain stupid.
Hard-on = excessively strict or severe, not an actual erection.
When the words are combined, you get: a person who thinks they are in charge and are excessively strict and/or severe, however that person is or is border-line mentally handicapped and every ignores him.
Ted: No talking during break time! My shoes hurt!
John: Ted, you're such a Tard-on!