glib tongue; smooth talk; deceptive eloquence
Idiomatic expression referring to someone who talks smoothly and deceptively, using just the tip of the tongue (舌先) and a small fraction (三分) of effort to talk their way out of something or deceive others.
Don't be fooled by his glib words.
I'm fed up with politicians' smooth-talking speeches.
口先 refers to superficial or insincere talk, while 舌先三分 emphasizes the deceptive ease and fluency of the speech.
甘言 means sweet or flattering words, often with an intent to deceive, but 舌先三分 focuses more on the glibness and trickery of the talk itself.
The expression combines 舌先 (tip of the tongue) and 三分 (three parts, a small fraction), suggesting that with just a small part of the tongue one can talk deceptively. The exact origin is uncertain, but it is a set phrase in Japanese.