noun
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.
彼の舌先三分の言葉に騙されないでください。
Don't be fooled by his glib words.
政治家の舌先三分の演説にはうんざりだ。
I'm fed up with politicians' smooth-talking speeches.
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.