Janus (Roman Mythology)
Janus was one of the most ancient and dignified of Roman Gods. Whereas nowadays, "Janus-faced" is used to mean deceitful or hypocritical, Janus was originally a kind and protecting God. Mythically, he guarded doorways. Thus one face inspected those who entered, whilst the other bade farewell to those who exited. He was responsible for domestic security and the safety of passangers.
As Rome grew in power and importance, so did Janus. He became the God of all thresholds. This included; gates and ports. He also became the God of passages which covered birth (as the first great passage of life), and the beginning of everything - such as the beginning of the Year. This is why on the Roman calender the first month was named after Janus - Januarius.
However, Janus soon lost his status and power. When the Romans assimilated Greek myths into their own, they discovered that the Greeks had no equivalent God. Janus was demoted from elder God to a King that was deified upon his death. He was made Heaven's gatekeeper, whilst Saturn took most of his duties as threshold God. This was because Saturn was now identified with the Greek God Cronus.
Despite entirely positive accounts of Janus throughout mythology, the negative view of Janus (and the resulting insult - 'Janus-faced') originated in 1711 when Anthony Ashley Cooper, the third Earl of Shaftesbury wrote Characteristicks sic of Men, Manners, Opinions, Times. In this text he wrote about the "Janus-face of some writers, who with one countenance force a smile and with another show nothing beside rage and fury"