means the quality of being genuine and honest, meaning exactly what you say without pretense or deceit.

from From Latin 'sincerus,' meaning 'pure, clean, sound' — originally of things unmixed and untainted, like wine without water. There's a charming folk story that it comes from 'sine cera,' 'without wax,' supposedly describing honest sculptors who didn't hide cracks in their marble with wax — but scholars reject this; the 'wax' tale is too neat to be true. The real root is murkier, the second part possibly tied to a word for 'growth,' suggesting something that grew whole and unspoiled. It entered English in the 1500s, already carrying its sense of unfaked feeling.