An early and still the most current sense of deprecate is “to express disapproval of.” In a sense development still occasionally criticized by a few, deprecate has come to be synonymous with the similar but etymologically unrelated word depreciate in the sense “belittle”: The author modestly deprecated the importance of his work. In compounds with self-,deprecate has almost totally replaced depreciate in modern usage: Her self-deprecating account of her career both amused and charmed the audience.
1620s, "to pray against or for deliverance from," from Latin deprecatus, past participle of deprecari "to pray (something) away" (see deprecation). Meaning "to express disapproval" is from 1640s. Related: Deprecated, deprecating.