Tae Soo Do was an early name for taekwondo. As the early kwans were coming together in the 1950s to develop a unified style of martial art for Korea (see: Taekwondo History), one of the questions that needed to be answered was what to call the new martial art. Some of the kwans had previously used the name Kong Soo Do as the name of their style; essentially, however, this name simply means "Korean karate". The kwans needed a name that reflected all their styles however.
As they debated a new name, the name Tae Soo Do was an early favorite. Choi Hong Hi however favored the name taekwon-do, in part because it sounded remeniscent of the ancient Korean martial art taekyon. By the mid 1960s Choi had successfully lobbied to have the name taekwon-do adopted instead.
Tae Soo Do is also the name of a style of Hwa Rang Do.