The idiom **耿耿于怀** is formed through a classical Chinese grammatical structure and vivid metaphorical imagery. It literally breaks down as **耿耿** (*gěnggěng*), describing something bright, shining, or—more pertinently—a persistent, pebble-like object that is hard and unsettling, and **于怀** (*yú huái*), meaning "in the heart/mind." Thus, the phrase paints a picture of a small, hard, glowing annoyance permanently lodged within one's bosom, which over time evolved into its modern figurative meaning: to harbor a persistent sense of resentment, bitterness, or preoccupation with a past grievance that one cannot forget or let go of. Its formation reflects a common pattern in classical Chinese where a reduplicated descriptive term is followed by a prepositional phrase to create a durable emotional metaphor.