serpentine, one who is especially cunning or shrewd, especially in conversation. One who has a way with words that can confuse you into agreeing to do something stupid.
Man, Bob sure is ligacious. We started off talking about birth control, and by the end of the conversation he had convinced me to have sex with him.
A word describing someone who is very talkative, and is also used as a synonym for fluent. Mike Doughty uses it ironically in his 2009 song "Pleasure On Credit".