Doodledog
I'm struggling to think of how it can be used as a noun - wouldn't that be 'difficulty'?
It can be a noun as in a class of people: The poor, the difficult, the outcast.
Also that which is difficult. He has an insatiable appetite for the difficult and the novel.
It's also a rock climbing term for a climb designated difficult: A Difficult on a small gritstone outcrop in Yorkshire may be much more demanding..than a Very Difficult in North Wales.
All examples from OED.


