آب

آب traces to the Proto-Indo-European root *h₂ep- ("water"), surviving across the Indo-Iranian branch. Sanskrit "ap" (water), Avestan "āp" (water), and Old Persian "āp-" all share the same source. The same root reaches into English place names through Latin: "Avon" (a Celtic river name from this root) and the suffix "-apa" in old European hydronyms. Within Iran, آب structures the landscape and the calendar. The traditional qanāt system (underground water channels) shaped where Persian cities settled. Pre-Islamic Persians celebrated آب with seasonal festivals like Tirgan, when people splash each other with water at midsummer. The word still carries that weight in modern Iran. The Zoroastrian phrase "ash kasi rikht" (someone's water spilled) is still used metaphorically when something good is wasted.