A
hypocrite; a person inwardly
corrupt or wicked but outwardly or professedly virtuous or
holy.
The phrase comes from a simile offered by Jesus in Matthew 23:27, w3rd. At that time, a Jewish sepulcher was routinely whitewashed to indicate that it was a tomb, so that people would not approach too closely. Jesus dissed the Pharisees by saying that they were like whited sepulchers, appearing beautiful and pure and rad on the outside but filled with the bones of the dead and general "uncleanness" inside. The comparison was designed to point out the hypocrisy of the Pharisees, and evidently it was such a powerful image that it was adopted into general speech to describe anyone who seemed (or made an effort to appear) pure and clean while secretly hiding an inner corruption. The phrase first appeared in print in 1582. Give it up for JC's mad metaphor skilz yo.
Gee, thanks, www.m-w.com!