吧唧嘴 (smacking one's lips while chewing) is one of the most-cited bad table manners in mainland Chinese etiquette guides, alongside 翘二郎腿 (crossing legs at the table) and 用筷子指人 (pointing with chopsticks).
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Both senses are colloquial. The mouth-smacking sense is sometimes pronounced 吧唧 with a neutral tone on the second syllable. The onomatopoeia sense is the squelchy step/kiss/slap kind of sound.
