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"letters entitling the bearer to certain credit or confidence," 1670s, from Medieval Latin credentialis, from credentia (see credence). Probably immediately as a shortening of letters credential (1520s, with French word order); earlier was letter of credence (mid-14c.).
"that which entitles to credit," 1756, probably a back-formation from credentials. Earlier in English as an adjective, "confirming, corroborating" (late 15c.). As a verb, "provide with credentials," by 1828 (implied in dredentialed).