Literally, "holy simplicity", an expression of astonishment at another person's naivety or naïvety (simple mindedness).

These are said to have been the dying words of John Huss (1373-1415), Bohemian religious reformer and martyr, provoked by the sight of a simple peasant adding wood to the fire around the stake where Huss was being martyred or burned to death for his religious beliefs.