troop: [16] Troop was borrowed from French troupe (acquired again as troupe in the 19th century). This appears to have been a backformation from troupeau ‘flock, herd’, a diminutive formation based on Latin troppus. And troppus itself may have been of Germanic origin. By the time the word reached English it was already being applied to a ‘group of soldiers’, and its plural was being used as a collective term for ‘soldiers’. => troupe[troop etymology, troop origin, 英语词源]