刻骨铭心 literally means 'to carve into bone and engrave upon the heart' — a vivid metaphor expressing something so profoundly moving or impactful that it becomes permanently etched in one’s memory and emotions. Each character contributes to this intensity: 刻 (to carve), 骨 (bone), 铭 (to inscribe), and 心 (heart). Unlike ordinary memories, 刻骨铭心 experiences are emotionally overwhelming — often tied to love, loss, trauma, or triumph — and resist fading over time.

This idiom is used almost exclusively in formal or literary contexts, especially when describing deep emotional imprints: a first love, a wartime experience, or a life-changing mentorship. It carries strong connotative weight and is rarely used for trivial matters. While grammatically an adjective, it typically modifies abstract nouns like 经历 (experience), 记忆 (memory), or 爱 (love), and often appears after 是 or as a predicate complement.