Morituri te salutant

"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.