From Brewer: We had some posts challenging this derivation, but I am feeling lazy about citing them.
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.)

This derivation was in my first year Latin book.