Late 17th century; earliest use found in John Ray (1627–1705), naturalist and theologian. Apparently imitative. With use as a command to a horse to stop or stand still compare German brr, †burr, Icelandic prr, Danish prr, pru, pruhh, ptro, ptru, etc., Swedish pro, prro, proh, ptro, etc., French brrr, all in the same sense. With use as a call to a cow or calf compare also Scottish Gaelic pruidh, in the same sense (of uncertain origin, perhaps from English).