When you are insincere, you say kind or warm things — “I’m so happy for you,” “That must be so hard” — but you do not really feel them. You are putting on a face. People use it a lot about smiles, compliments, and apologies: an insincere smile sits on the mouth and never reaches the eyes. It is a fairly strong, slightly careful word; in chat, friends more often say you don’t mean it, or that you are being fake. Though actually, it is not the same as lying about facts. A liar might invent a story. An insincere person can tell the truth about what happened and still fake the feeling. Teachers and parents reach for it when manners are only for show.