triāriīm pl (genitivetriāriōrum); second declension
(military, plural only) the third and last line in the Roman army (after, respectively, the prī̆ncipēs and the hastātī)
27 BCE – 25 BCE, Titus Livius, Ab Urbe Condita7.23:
Ab Romanis necopus intermissum—triarii erant, qui muniebant—etab hastatis principibusque, quipro munitoribus intenti armatique steterunt, proelium initum.