early 12c., from Old French saint, seinte "a saint; a holy relic," displacing or altering Old English sanct, both from Latin sanctus "holy, consecrated" (used as a noun in Late Latin; also source of Spanish santo, santa, Italian san, etc.), properly past participle of sancire "consecrate" (see sacred). Adopted into most Germanic languages (cf. Old Frisian sankt, Dutch sint, German Sanct).
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.
Applied widely to living things, diseases, objects and phenomena, e.g. Saint Bernard, the breed of mastiff dogs (1839), so called because they were used by the monks of the hospice of the pass of St. Bernard (between Italy and Switzerland) 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.