"Morituri te salutant" was a Latin phrase that the Tenth Doctor recited when faced with seemingly certain death at the hands of Lucius Petrus Dextrus and his soldiers. Petrus Dextrus, hearing a TARDIS-translated version, thought it a "Celtic prayer". (TV: The Fires of Pompeii)
Behind the scenes[]
Though not explained in Fires, the phrase means something like "Those who are about to die salute you". Especially given his next line to Quintus — "dignity in death" — the Doctor is implied to be admitting his defeat to Petrus Dextrus, although he is actually fibbing to reveal the augur's stone arm.