English word "sincere" comes from two Latin words:

sine = without
cere = wax.

In Ol' Rome, potters used wax to cover up cracks and flaws in pottery.

Potters who wanted folks to know they did not follow this practice had the sign in the shop declaring , "Sine Cere" - Without Wax, no hiding of imperfections or flaws.

In Greek the word is:

aploteti = "sincerity"

It includes not only the idea of sincerity, but also doing more than is expected: Promising less but giving more than expected.