Refers to a person who is skilled at saying pleasing things, often with a nuance of insincerity or manipulation. Not a common everyday word; more often encountered in literary or historical contexts.
He is a flatterer, always ingratiating himself with the boss.
口先上手 emphasizes being glib or smooth-talking, often with a negative connotation, similar to 上手者 but more focused on verbal skill.
Compound of 上手 (じょうず, 'skillful') and 者 (もの, 'person'). The exact historical derivation is uncertain, but the word literally means 'a skillful person' and developed the specific sense of a flatterer.