Why is the ninth month called September?
c.1500, "prayer to avert evil," from Middle French deprécation, from Latin deprecationem (nominative deprecatio), from past participle stem of deprecari "plead in excuse, avert by prayer," literally "to pray (something) away," from de- "away" (see de-) + precari "pray" (see pray). Sense of "disapproval" is first attested 1610s.
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.