This article explores Adriatic irredentism, a complex political, cultural, and social movement, by specifically analyzing the unique role it played in the legitimization of Italian territorial claims over “language frontiers” such as Trieste and its hinterland. Through a close reading of first-hand sources, it examines how Italian irredentist intellectuals, public press, and associations purposefully utilized anti-Slav and anti-German arguments to shape public perception of both the Italian nation as well as Trieste's Italian identity or “Italianità.” Although recent historiographical interpretations have emphasized continuities in local understandings of “Italianità,” this article examines the discontinuities in the debate over its identity. It suggests that although Italian identity was first conceived as an expression of cultural and linguistic autonomy within the broader intellectual framework of Adriatic multi-nationalism, this idea gradually vanished amidst the structural crisis triggered by the Ausgleich or Compromise of 1867 and then inexorably faded on the eve of the Great War. Thus, notions of Italian national identity took an exclusionary and sometimes xenophobic meaning that was publicly used by a wide set of political actors to justify the territorial reincorporation of the “unredeemed” land within the borders of the new Italian state. The fascist regime, especially, utilized Italianness to further its aggressive and chauvinist agenda toward the Adriatic borderland. Consequently, Italian language and culture became instruments as well as symbols of repression and imperialism that were used to fulfill the regime's ambitions of “fascistization” of the Slavic population living in the region.