Someone who seems like an upright citizen, but really has no spine.
When the old lady tripped and fell, Jim rushed over to 'help' her- by relieving her of the weight of her purse. What an imposture.
Deception: The act of pretending to be somebody else in order to trick people, or an occasion on which this is done.
Imposture is what imposters do.
The act of imposture is against the law.