As you say, a specific definiton of exactly what dignity is would be hard to think of.
In my eyes, dignity is a label given for some of the very basic optimistic emotions a human could display. This is slightly because people often refer to it as the last 'possession', based on the common phrase "At least he still has his dignity". It's the fine line between life being worth living and it not. It is a very forceful driving force that keeps us going
Therefore, it is not greed-driven, like honour or pride, but rather something much more personal and individual. Therefore, I suppose that dignity is different to everybody. I don't believe that it is holding your head up high, but rather getting up in the mornings, no matter what. It's about, no matter what your consequences, trying to make the most of things, or at least putting some effort in. It is what many people live for, even if they have lost everything else.
I would also say that it is more common in the 'innocent' or 'morally righteous'; those who would not usually act for their own sake. It's not to do with living up to the expectations of others around you, but remaining true to youself.

I'm sorry if I've rambled on a bit, but, to be honest, I've never really thought about dignity much before, so my thoughts are a bit muddled. Maybe somebody else will be able to make some sense out of them.

Someone who stands by their principles no matter what the obstacles.

Would that definition stand if a persons principles were evil? Not to be argumentative, but I think Hitler would fit that definition!

Dignity does not explore the character it defines his persona..You could say Mark Anthony, but do you admire his politics ? Its not judging his motives or his principles.

To my mind, dignity is about occupying some high moral or ethical stance within a given context, where the rules depend on that context. In Remains of the Day, the butler's dignity comes out of senses of duty, pride and loyalty. We as readers can judge him as quite undignified by other standards. I think the point of the novel is that dignity removes freedom since we defer our will to externally derived ethical codes. The butler realises in the end that he would have been happier had he instead followed his own desires. It's quite existential: the notion of bad faith is at its core.

Usually dignity is something that is seen by others. The point of being dignified is to be judged as being dignified, so it is essentially conformism to the rules of the judges. As we're well aware of ourselves as objects for others, it's a very tricky thing to shake off.