He said he was sorry, but I could tell that he was being 'insincere'.
an 'insincere' apology
smiling 'insincerely'
(..) I wish I could be more like you. You mustn't flatter me, Melanie, and say things you don't mean. Nobody could accuse Melanie of being insincere. Could they, my dear? Then she's not like you, is she, Ashley? Ashley never means a word he says to any girl. Oh, why, Charles Hamilton, (..)1939 Gone with the Wind