Patronymically from Ammown; an Ammonite or (the adjective) Ammonitish -- Ammonite(-s).
see HEBREW Ammown
H5984. Ammoni
עַמֹּנִי עַמּוֺנִי, adjective, of a people Ammonite; — absolute masculine singular עַמּוֺנִי as substantive Deuteronomy 23:4 an Ammonite (generic), so עַמֹּנִי Nehemiah 13:1; הָעַמּוֺנִי of individual 1 Samuel 11:1-2, 2 Samuel 23:37 = 1 Chronicles 11:39; Nehemiah 2:19, so הָיַמֹּנִי 2:10; Nehemiah 3:35; feminine singular הָעַמֹּנִית 1 Kings 14:21 2 Chronicles 12:13; 1 Kings 14:31, so הָעַמּוֺנִית2 Chronicles 24:26; הָעַמֹּנִי as substantive collective Ezra 9:1; masculine plural as substantive עַמֹּנִים 1 Kings 11:5, ׳הָע Deuteronomy 2:20; Nehemiah 4:1, ׳הָעַמּוֺ2 Chronicles 26:8 (on 20:1 see מְעוּנִים); feminine plural as adjective עַמֳּנִיֹּת 1 Kings 11:1, so Nehemiah 13:23 Qr, Kt עמוניות.