Origin uncertain; compare the earlier proo.
- A command to a donkey or mule to move faster.
1917, Charles S. Brooks, There's Pippins and Cheese to Come, New Haven, Conn.: Yale University Press, OCLC 765186, page 38:
- The window is handed in [to a wicker carriage]. Her feet are wound around with comforters against a draft... Her ample bag of knitting is safe aboard... Proot! The donkey starts.
- Brod (Standard German based orthography)