Baretti: A dictionary of the English and Italian languages
Sincér-o, adj. sup. Sincér-íssimo,
1. sincere, true, open, plain, ingenuous; 2. inartificial, natural; 3. in good health; 4. genuine, not counterfeit; 5. secure, safe; 6. not deteriorated by marriage with an inferior race. — d'errore, innocent. Voce sincera, a legitimate word. Ragione sincera,an evident reason; adv. Sincér-aménte, sup.Sincér-issimaménte .
Florio: a worlde of wordes, or most copious, dictionarie in Italian and English