As defined on Wiki, ‘siwash’ is derogatory, and was ethnic of North West Indian origin. In modern times, say from 1920 on, the word became generic, applying especially to white guys who were down on their luck and therefore had to live in the woods without shelter. The average guy believes that his blanket is for warmth. The siwash knows better. The siwash sets a log on fire and lies down next to it. In this situation, your blanket is not for warmth, rather it is your protection from flying sparks. Only the siwash knows this.
In the following link will also be found high ass muckitymuck.