The word "诚惶诚恐" is formed by the juxtaposition of two nearly identical phrases, "诚惶" and "诚恐", each consisting of the adverb "诚" meaning "sincerely" or "truly" modifying a character denoting fear, "惶" for "fearful" and "恐" for "afraid", thereby creating a compound that emphasizes intense apprehension through repetition and synonymy.