-də(r) noun
Etymology: rough (I) + rider
1. : one who breaks horses to the saddle or who is noted for or accustomed to riding little-trained horses ; specifically : a trooper in the British cavalry who is assigned as a horse trainer
2.
a. : an irregular cavalryman
b. usually capitalized : an officer or enlisted man in the 1st United States Volunteer Cavalry regiment in the Spanish-American War composed mostly of western cowboys and hunters and eastern college athletes and sportsmen commanded by Theodore Roosevelt