The word 刻骨铭心 is formed by combining two parallel verb-object phrases: "刻骨" (carve into bone) and "铭心" (inscribe on the heart). Each phrase uses a verb ("刻" meaning to carve, and "铭" meaning to inscribe) followed by a noun ("骨" for bone and "心" for heart), creating a literal meaning of engraving onto bone and heart. This structure emphasizes depth and permanence through metaphorical imagery, resulting in a compound idiom that conveys something unforgettable.