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 hid your face, hid your emotions, hid your true self. Without wax, your face, your emotions, and your true self would be exposed to the world: you would become sincere.
