Which term is characterized as fluent and voluble but insincere and shallow?

The term that describes someone who is fluent and voluble but insincere and shallow is "glib." This word often refers to speech that appears smooth and persuasive on the surface but lacks depth or sincerity. A glib speaker might come across as charming and articulate, yet their words often fail to convey genuine thoughtfulness or authenticity. This characteristic can manifest in contexts where an individual is attempting to impress others without having a substantive understanding or commitment to what they are saying.

The other terms do not convey this specific combination of qualities. "Scathing" refers to something that is harshly critical or scornful, which does not imply fluency or insincerity. "Feckless" denotes a lack of initiative or strength of character, which diverges from the focus on verbal fluency and smoothness. "Exigent" describes situations that demand immediate action or urgent attention, which is unrelated to the manner of speaking or the qualities associated with glibness. By focusing on the traits of fluency combined with superficiality, "glib" stands out as the correct choice.