"七嘴八舌" is formed by directly combining the numeral "七" (seven) with "嘴" (mouth) and the numeral "八" (eight) with "舌" (tongue), creating a literal sequence meaning "seven mouths and eight tongues". This juxtaposition uses exaggerated counts to depict a scene of numerous people speaking simultaneously, thus conveying the idea of a chaotic or noisy conversation.