耽于 is a formal, literary verb meaning 'to indulge in' or 'to become excessively absorbed in' something—often with a negative connotation of neglecting duties or responsibilities. The character 耽 (dān) originally means 'to delay' or 'to linger', implying prolonged, unproductive engagement; 于 (yú) is a preposition indicating the object or domain of that indulgence (e.g., 'in pleasure', 'in fantasy'). Together, they form a compact, classical construction used primarily in written Chinese, essays, historical narratives, or formal speeches—not in casual conversation.

This phrase typically governs abstract nouns like 逸乐 (pleasure), 幻想 (fantasy), 酒色 (wine and women), or 声色 (music and sensual pleasures). It carries moral or rhetorical weight, often warning against excess or self-deception. Because of its literary register, it rarely appears in spoken Mandarin unless quoting classical texts or adopting an elevated tone.