The word "心惊胆颤" is formed by pairing two descriptive phrases: "心惊", where 心 means "heart" and 惊 means "startled", and "胆颤", where 胆 means "gallbladder" (associated with courage) and 颤 means "tremble". Each phrase depicts a physical reaction to fear, the heart being alarmed and the gallbladder shaking, which together emphasize a state of intense terror through parallel structure.