Why is the ninth month called September?
early 14c. (late 12c. as a surname), "mercy, tenderness, pity," from Old French piete "piety, faith; pity, compassion" (12c.), from Latin pietatem (nominative pietas) "dutiful conduct, sense of duty; religiousness, piety; loyalty, patriotism; faithfulness to natural ties," in Late Latin "gentleness, kindness, pity;" from pius "kind" (see pious). Meaning "piousness" attested in English from c.1600. Also see pity (n.).
Lat. pietas, properly honour and respect toward parents (1 Tim. 5:4). In Acts 17:23 the Greek verb is rendered "ye worship," as applicable to God.