Is it ensure, insure, or assure?
1540s, "body of soldiers," from Middle French troupe, from Old French trope "band of people, company, troop" (13c.), probably from Frankish *throp "assembly, gathering of people" (cf. Old English ðorp, Old Norse thorp "village," see thorp). OED derives the French word from Latin troppus "flock," which is of unknown origin but may be from the Germanic source.
1560s, "to assemble," from troop (n.). Meaning "to march" is recorded from 1590s; that of "to go in great numbers, to flock" is from c.1600. Related: Trooped; trooping.