Speculum Humanae Salvationis
"Though you may take my life, you will never take my complete apathy."
These are the result of a medieval obsession with Ars Moriendi, or the art of dying well. Medieval people were understandably preoccupied with death and the afterlife, since they were constantly dying of pretty much everything. But giving in to agony and allowing your face to contort into an outward reflection of one of the five temptations (disbelief, despair, impatience, pride, and avarice) was a sure way to find yourself bathed in eternal hellfire. On the other hand, displaying the calmness of Christ on the cross was a one-way ticket to the pearly gates.
Since the whole thing was about dying a Christly death, nobody was better at ignoring their own murder than the priesthood. Here's Saint Peter of Verona, for instance, who didn't even let a simultaneous chest-stabbing and head-splitting interrupt his prayer time:
"I'll be with you in a moment, giant cleaver."