1540s, "body of soldiers," 1540s, from French troupe, from Old French trope "band of people, company, troop, crowd" (13c.), a word of uncertain origin, perhaps from Frankish *throp "assembly, gathering of people" or another Germanic source, perhaps related to Old English ðorp, Old Norse thorp "village" (see thorp). OED derives the French word from Latin troppus "flock," which is of unknown origin but also might be from the proposed Germanic source. Of groups of animals from 1580s. Specifically as "a subdivision of a cavalry force" from 1580s; of Boy Scouts from 1908. Troops "armed forces" is from 1590s.
"professional leader of a mercenary troop," 1794, from Italian condottiere, from condotto "to conduct," from Latin conducere "to lead or bring together" (see conduce).
"group of fish," late 14c., scole, from Middle Dutch schole (Dutch school) "group of fish or other animals," cognate with Old English scolu "band, troop, crowd of fish," from West Germanic *skulo- (source also of Old Saxon scola "troop, multitude," West Frisian skoal), perhaps with a literal sense of "division," from PIE root *skel- (1) "to cut." Compare shoal (n.2)). For possible sense development, compare section (n.) from Latin secare "to cut."