Some persons have dual character, one normal and the other abnormal.Such characters are said to be janus-faced or janus-like. The word janus-like is an adjective synonymous with the words duplicitous, two-faced. This word came into usage in English vocabulary in 1682.
Janus is a double-faced Roman God. In Roman mythology, Janus was considered as god of beginning, gates, transition, time and duality. The month January is named after him. He was invoked as the first of all gods in ancient liturgies, as the Hindus worship God Vinayaga as the first god. The word janus is a contronym , a word with apposing meanings. A janus-faced person has deliberate deceptiveness , especially by pretending one set of feelings but acting under the influence of another. His behaviour is often fake.
Some persons act like angels under authority but like devils in the absence of authority. They are janus-faced. Such a behaviour is common among school children; they are disciplined in the presence of their teachers but rebellious when their teachers leave the class. We commonly come across janus-faced persons among our relations, friends and colleagues.
In science,the relations denoted by the generalised master formula
[ F, G†] = 1
is called janus-faced commutation relation; the eigenstate G occurs as a short term solution and the eigenstate F occurs as a steady state solution.