Yachiru means several things. For one, it's the first name of the Lieutenant of the 11th Division, Yachiru Kusajishi. Kenpachi Zaraki gave her that name after the one person he admired the most; namely Retsu "Yachiru" Unohana, who gave herself the nickname "Yachiru", since she mastered all forms of combat. And finally, Yachiru itself means (according to the wiki) "Eight thousand/many flows" or "Eight thousand/many exiles".