The word "嘀咕" is formed by combining two characters, each containing the mouth radical (口) to signify relation to sound or speech. The character "嘀" incorporates the phonetic component "啇" to suggest pronunciation, while "咕" uses the phonetic component "古" for its sound. Together, they create a disyllabic word that phonetically imitates the act of whispering or muttering, with the radicals consistently indicating its association with vocalization.