This article is from the alt.usage.english FAQ, by Mark Israel misrael@scripps.edu with numerous contributions by others.
"Sincere" is sometimes said to derive from Roman quarrymen's
temporarily concealing imperfections in marble blocks by rubbing wax
on them. On its AOL message board, Merriam-Webster Editorial
Department writes: "The theory that 'sincere' ultimately derives
from Latin "sine cera", meaning 'without wax', is a popular one;
unfortunately, there is no evidence to support it. A far more
likely origin, in our view, is that the Latin word "sincerus"
derives from "sem-" ('one') and "-cerus" (akin to Latin "crescere",
meaning 'to grow')."
 
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