The adj sincere has 2 senses (first 1 from tagged texts)
1. (6) sincere -- (open and genuine; not deceitful; "he was a good man, decent and sincere"; "felt sincere regret that they were leaving"; "sincere friendship")
2. earnest, sincere, solemn -- (characterized by a firm and humorless belief in the validity of your opinions; "both sides were deeply in earnest, even passionate"; "an entirely sincere and cruel tyrant"; "a film with a solemn social message")