In a thing of nought; a nothing—a thing which does not really exist, viz. your prosperity and power. Horns; symbols of strength (Deuteronomy 33:17; 1 Kings 22:11); the idea being derived from the wild bull, the strongest animal of their fauna.
Their boast was a consequence of the successful wars with the Syrians (2 Kings 14:25-28). The prophet proceeds to demolish their proud vaunt.