The word "眼中钉" is formed by combining three characters: 眼 (eye), 中 (in), and 钉 (nail), creating the literal meaning "a nail in the eye". This constructs a vivid metaphorical image where something as painful and intrusive as a nail in the eye represents a source of extreme irritation or hatred, directly deriving its figurative sense from the physical discomfort implied by the characters' meanings.