The phrase employs the noun μισθοφόρος (mercenary) as an attributive adjective modifying τριήρεις (triremes), a usage that personifies the ships as hired soldiers. This is a specific instance of the metonymic 'soldier-ship' metaphor found in Old Comedy, where the trireme is treated as a citizen-soldier paid for service, reflecting the contemporary political discourse in Athens regarding the professionalization of the navy and the criticism of demagogues (like Cleon) who allegedly treated the fleet as a source of employment rather than a state instrument. The construction is syntactically elliptical, omitting the verb of being or action, relying on the reader to supply the sense of 'trières that are mercenaries' or 'mercenary triremes'.