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The first three words of the Aseres Hadibros "Onochi Hashem Elokecha" = 173
According to Rokeach (excerpt here), the fourth blessing of Musaf on (weekday) Rosh Chodesh contains 173 words. (Though by my count, Nusach Ashkenaz has 174, not counting ולכפרת פשע which is recited sometimes, and Nusach Sefard has even more.)
Avraham Avinu was 173 years old when Yaakov and Eisav turned 13.
173 rabbis, headed by the Aruch Laner, protested the Brunswick conference. Source: The Jewish Encyclopedia.