E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
An Austrian soldier. So called because he wears a white coat. Similarly, an English soldier is called a red-coat. In old Rome, ad saga ire meant to become a soldier, and tumere sagum to enlist, from the sagum or military cloak worn by the soldier, in contradistinction to the toga worn by the citizen in times of peace.