Actually, it is pronounced wa-shee-choo. I hear it enough from the Lakota up here.
Side note, when the folks from Pine Ridge jumped in their cars to come to the opening of Dances here in Rapid City they did not know it was gonna be a comedy. But when they heard all the men speaking "woman's language Lakota" they laughed their heads off. Turns out the person they hired to translate the lines into Lakota only translated into woman's language so the men are speaking as if they were gay.