Which term specifically relates to a person who speaks in a smooth but insincere manner?

The term that specifically relates to a person who speaks in a smooth but insincere manner is "glib." This word describes someone who talks effortlessly and fluently, often without enough thought behind their words, which can give the impression of being untrustworthy or insincere. A glib speaker may charm an audience with their ease of communication, but their words may lack depth or sincerity.

Goad refers to provoking or annoy someone to stimulate some action or reaction, which does not capture the idea of a smooth talker. Exculpate means to clear someone from blame or fault, which is unrelated to the manner of speaking. Inquisitive describes someone who is curious or eager to learn, focusing on a desire for knowledge rather than a style of communication. Thus, glib is the most apt choice for describing someone who speaks sweetly and persuasively but may not be honest or sincere.