kid on the square

Etymology
Dates back to the early 20th century. Later popularized by in his 1983 song "Kidding on the Square" on the album Middle Class White Boy (described in the liner notes as "joking with serious intent") and later by  describing the negative reaction of  to one of his jokes in the 2003 book 

Verb

 * 1) To be joking, but at the same time really mean it.