The word "嘟囔" is formed by combining two phono-semantic characters: "嘟" uses the mouth radical (口) to denote sound-related meaning and approximates a mumbling noise, while "囔" similarly incorporates the mouth radical to suggest nasal or muttered speech; together they create an onomatopoeic compound that directly mimics the act of muttering, thus conveying the meaning of speaking in a low, indistinct voice.