Formal:
* Glib: This word implies a smooth and effortless fluency, often used to mask a lack of genuine thought or feeling.
* Articulate: While generally positive, "articulate" can be used ironically to highlight someone's smooth delivery despite a lack of sincerity.
* Eloquent: Similar to "articulate," "eloquent" can carry a sense of insincerity if the speaker's words seem overly polished and lacking in genuine emotion.
Informal:
* Slick: This word suggests a polished, superficial surface that hides a lack of depth or sincerity.
* Smooth-talking: This phrase highlights the persuasive and fluent nature of speech, often used for someone who manipulates with words.
* Silver-tongued: Similar to "smooth-talking," this term implies a talent for charming and convincing others, even if the intent is not sincere.
* Prattle: This word implies a rapid flow of words without much substance or meaning, often used to describe someone who talks incessantly without thinking.
Other options:
* Superficial: This word emphasizes the lack of depth and sincerity in someone's communication.
* Insincere: This word directly addresses the lack of genuine feeling behind the speech.
* Deceptive: This word implies a deliberate attempt to mislead through speech.
The best word to use will depend on the specific context and the nuances you want to convey. You could also use a combination of words to create a more precise and evocative description.