A very unpleasant or stupid person. In case you are not yet convinced that someone is a cunt noun, the related adjective, "cuntable", can be applied.
"He has been a very successful businessman as a former hedge fund manager and a CEO at many other companies, yes, but ,if you get closer to him, then you can see he's such a heartless and merciess idiot. In other words, he's a cunt noun."