You should read wiki first before you make a statement there Rockman and see where our term came from. I will save you from searching since you couldn't be bothered to do in the first place.

Name:

Despite some members' claims throughout its history that the name "Imaginary Numbers" is a mathematical joke, it would seem that the name can originally be sourced to the game Front Mission 3, released for Playstation in 1999 by Squaresoft. In this game, Imaginary Numbers are a form of genetically engineered super soldiers who are superior to the previous form of soldier known as Real Numbers.