忠于 (zhōng yú) is a prepositional phrase meaning 'loyal to' or 'faithful to'. It expresses steadfast devotion, allegiance, or commitment toward a person, group, principle, cause, or ideal. The character 忠 (zhōng) means 'loyalty' or 'fidelity', while 于 (yú) is a classical preposition indicating direction or target — here, the object of one's loyalty. Together, they form a formal and somewhat literary construction, commonly used in written Chinese, speeches, official documents, and moral discourse.

This phrase often appears in contexts emphasizing duty, ethics, or ideological commitment — for example, being loyal to one’s country, a leader, a promise, or core values like truth or justice. It carries positive connotations of integrity and steadfastness. While grammatically it functions like a preposition, it must be followed by a noun or noun phrase (e.g., 忠于祖国, 忠于职守). It is not used predicatively on its own (i.e., you wouldn’t say *他忠于* without specifying the object).