Collegiate Shag is another partnered dance from the Swing Era that is typically done to faster tempos. The posture for Shag is more upright (like Balboa) and uses footwork patterns of steps and hops. There are 3 different rhythms used in Collegiate Shag: Single, Double, and Triple. The rhythm you are dancing has to do with the number of slows (step-hops) that happen in one basic followed by a quick-quick.
Unfortunately, the origins of Shag are not very clear but it is thought to have evolved from the Foxtrot. Dance historians state that it started in the southern U.S. and was known as a "street dance", which basically means it wasn't created in any kind of dance studio. The dance wasn't universally known as Shag, as others also called it the Flea Hop. The "Collegiate" part of the name describes the college age dancers that brought a youthful energy to the dance.
Other Shag dances that became popular in the 1940s and 1950s are Carolina Shag and St. Louis Shag. These other forms of Shag have a very different look and feel from Collegiate.