The name "Shag" encompasses a number of swing dances originating from early 20th century America. It is thought to derive from the term "shagger", which was supposedly a nickname for late 19th century vaudeville performers, who were known to dance the Flea Hop.
The Shag was popular in the late 1920s, particularly with college students, and was primarily danced to fast Ragtime Jazz music. In the 1920s-30s, there were many Intercollegiate Dance Contests held, with specific Shag Divisions. Virginia Beach was a hot-spot for the Shag as some eastern cities banned the dance.
By the 1930s, there were arguably a hundred or more variations of the dance, which differed depending upon geographic region. These variations were later generalized into three categories: single, double, and triple shag. The different names denote the number of 'slow' (e.g. step, hop) steps performed during each basic. The slow steps were then followed by two 'quick' steps (e.g. step, step).
The Collegiate Shag originated in the South (possibly in New Orleans) and has been known at times as the "Flea Hop". The Carolina Shag was an offshoot of the Collegiate Shag with Charleston influences, and today, it is considered more of a Swing dance form than a Shag dance form. The Saint Louis Shag is a speed-dominated/competition shag, and differs from the Collegiate, Carolina and Murray Shags. The Balboa (8-count) was a more subdued, smoother variation of the Collegiate shag (6-count).
Today, the term "Collegiate Shag" is most often used in reference to a kind of double shag that is believed to have originated in New York during the 1930s. During the Swing era, the dance would not have been called the "Collegiate Shag" - it is thought that the word "collegiate" was added later as a marketing ploy to attract college-aged dancers to certain studios. The name became more standard in the latter part of the 20th century, presumably because it helped to distinguish the dance from other contemporary dances.