Originally an adjective prefixed to the name of a canonized person; by c. 1300 it came to be regarded as a noun. Meaning "person of extraordinary holiness" is recorded from 1560s.

Saint, n. A dead sinner revised and edited. The Duchess of Orleans relates that the irreverent old calumniator, Marshal Villeroi, who in his youth had known St. Francis de Sales, said, on hearing him called saint: 'I am delighted to hear that Monsieur de Sales is a saint. He was fond of saying indelicate things, and used to cheat at cards. In other respects he was a perfect gentleman, though a fool.' [Ambrose Bierce, "Devil's Dictionary," 1911]

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Perhaps you have imagined that this humility in the saints is a pious illusion at which God smiles. That is a most dangerous error. It is theoretically dangerous, because it makes you identify a virtue (i.e., a perfection) with an illusion (i.e., an imperfection), which must be nonsense. It is practically dangerous because it encourages a man to mistake his first insights into his own corruption for the first beginnings of a halo round his own silly head. No, depend upon it; when the saints say that they--even they--are vile, they are recording truth with scientific accuracy. [C.S. Lewis, "The Problem of Pain," 1940]

Saint Bernard, the breed of mastiff dogs (1839), so called because the monks of the hospice of the pass of St. Bernard (between Italy and Switzerland) sent them to rescue snowbound travelers; St. Elmo's Fire "corposant" (1560s) is from Italian fuoco di Sant'Elmo, named for the patron saint of Mediterranean sailors, a corruption of the name of St. Erasmus, an Italian bishop martyred in 303.