The story goes that Roman sculptors who worked in marble would use wax to fill in flaws in their sculptures, or even to affix parts that had accidentally broken off. Of course, on a sweltering hot day the wax would become soft and the sculpture could become deformed or even break apart. The best sculptors would assure their clients that their sculptures were without wax, or, in Latin, "sin cera."
It sounds good, doesn't it? In fact, I was so amused the first time I heard it that I signed a couple of letters "Without Wax, Chris."
Unfortunately, it probably isn't true. Merriam-Webster Unabridged is free this week, and their entry contains the following:
Etymology: Middle French, from Latin sincerus, probably from sem- one + -cerus (from creare to create).
No clue why the regular Merriam-Webster entry would be slightly different:
Etymology: Middle French, from Latin sincerus whole, pure, genuine, probably from sem- one + -cerus (akin to Latin crescere to grow).
Either way, it suggests that the somewhat amusing stories about noses falling off marble statues are, in a word, insincere.