E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Sincere (2 syl.)
properly means without wax (sine cera). The allusion is to the Roman practice of concealing flaws in pottery with wax, or to honey from which all the wax has been extracted. (See Trench: On the Study of Words, lect. vii. p. 322.)