The English sincere is from the Latin “sine cera” — literally, “without wax.”
From that same root is… the Spanish, cera, for “wax” (not to mention, the Spanish sin, also meaning, “without.”) The c-r root is easy to see in both words.
However, what does “wax” have to do with “sincerity”? Well, wax was used to make masks — and makes you hide your face, hide your emotions, hide your true self. Without wax, your face, your emotions, and your true self would be exposed to the world: you would become sincere.