"Kagemusha" (the body double) is a person, during the Sengoku period (the period of warring states), employed by a busho (a Japanese military commander) as his straw man dressed in the identical clothes as his.
Kagemusha was employed for the purposes including disguising the death of a lord, or confusing enemies. Since photography was not available at that time and an assailant did not always know the face of the lord, it was an effective measure. Kagamusha is generally referred to as a "dummy". It is said that Nobushige (Yukimura) SANADA confused his enemies in the battlefield using as many as seven kagemushas.
Even in the present day, there is a view that Kim Jong-il of North Korea has his own kagemusha. Some people allege that while he is actually already dead, they are disguising the kagemusha as Kim Jong-il.
Derived from the meaning above, the term "kagemusha" is used today to refer to a celebrities' tactic to distract the attention of mass-media by using a secretary or a friend as their dummy in order to avoid interviews or being chased by paparazzi.