These four characters combine to create a powerful and intuitive image: an “oily mouth and a slippery tongue.” The greasiness of the oil implies something unctuous and insincere, while the slipperiness of the tongue suggests that the person's words are evasive, tricky, and cannot be trusted. The phrase perfectly captures the physical sensation of disgust one might feel when listening to a dishonest flatterer.
The term 油嘴滑舌 reflects a deep-seated cultural preference in China for sincerity (诚实 - chéngshí) and substance over superficial cleverness. While eloquence can be admired, speech that is too smooth or flattering is often met with suspicion. It's seen as a sign of a deceitful or frivolous character. The ideal is often the 老实人 (lǎoshí rén)—an honest, down-to-earth, and straightforward person. 油嘴滑舌 is the direct antithesis of this ideal. A Western learner might compare it to being “silver-tongued.” However, this comparison can be misleading. “Silver-tongued” can sometimes be neutral or even a compliment, describing a skilled orator or a charming speaker. 油嘴滑舌 has no such positive connotation. It is unequivocally negative. A closer English equivalent might be a “smooth-talking con man” or a “weasel,” capturing the inherent distrust and dislike for the person's character, not just their speaking style.
油嘴滑舌 is a common term in informal, everyday conversation. You use it to complain about or describe someone you find untrustworthy.