Janus was a deity in the Roman pantheon characterised by having two faces, and was the patron god of doors and new beginnings. January was named after him.
In Roman times, the shrine of Janus was often seen as ominious, for the doors of the shrine would be shut in times of peace but left open when the Romans went to war.
Janus in other references
The expression "Janus-faced" is also analogous to the expression "two-faced" - meaning a person with dual natures: in other words, a Deceitful and traitorous person. See