outrider

n person on a motor cycle (or, esp formerly, on horseback) escorting the vehicle of an important person （重要人物乘坐车辆的）摩托车警卫（或尤指旧时的骑马护卫）: The President's car was flanked by motor-cycle outriders. 总统的汽车由摩托车警卫护行.

outlet, outline, outlive, outlook, outlying, outmaneuver, outmanoeuvre (US outmaneuver), outmoded, outnumber, outplay, outpoint, outpost, outpouring, output, output device, outrage, outrageous, outrageously, outrank, outre, outrigger, outright, outrival, outrun, outsell, outset, outshine, outside, outside broadcast, outside lane, outside left, outside right player, outside line, outsider, outsize, outskirt, outskirts, outsmart, outspoken, outspokenly, outspokenness,