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The shuriken is a type of weapon which was employed by the ninja. Like tetsubishi (caltrops) and metsubishi (powder blowpipe), these small, thrown blades were used in desparate circumstances, most often with the intention to confuse the enemy and allow the ninja to escape. The actual deadliness of shuriken is generally overstated by Japanese and Western pop culture alike.
Originally, short swords (sashizue) such as the kogatana and wakizashi were used for this purpose, and the shuriken, in the form of a sharpened metal spike, was developed with the evolution of this technique.
Contrary to its common English name of "throwing star", very few shuriken were actually star-shaped; most were narrow, flat knives. Think about it -- how would you effectively conceal something that was sharp on all sides? Some cross-, ring-, and star-shaped shuriken did exist, however. Japanese popular culture also embraces the idea of shuriken as star-shaped.
The original meaning had nothing to do with stars, or even throwing, for that matter.
shuriken is written with three kanji characters:
手 (shu, or te) "hand"
裏 (ri, or ura) "back", "reverse side"
剣 (ken) "sword"
This name was originally applied to the technique of holding the short sword backwards in the hand, so that the blade faced the wielder and the handle faced the target. The ninja then snapped his or her wrist, turning the blade over and launching it, point-first, towards the opponent.
The name of this technique was eventually applied to a weapon used in this manner. When more compact, specialized weapons were developed for this purpose, they assumed the name shuriken.
Once, this technique was also known as "shiri-ken". "shiri" means "rear", or more literally, "buttock." Combine the disgrace of fleeing with this name, and it's not hard to imagine why this term lost out to "shuriken" in the long run.
Koujien 5th ed.
Shinseiki Visual Daijiten Electronic version. Sharp Color Electronic Dictionary PW-C5000.