Personal dignity is personhood itself, considered under the aspect of the respect one person owes another.
This is analogous to saying that goodness is being under the aspect of desire.
It's not ideal, since we're used to speaking of dignity as something a person has, and English at least doesn't really have the right adjective form. We say a thing "is good" while a person "has dignity." We might, before a generous audience, get away with saying a thing "has goodness," but to say a person "is dignified" simply doesn't mean the person is owed respect due to his personhood. We'd want a word like "dignitied," but... well, who really wants a word like "dignitied"?
On the up side, if we understand dignity, not as some quality attached or associated to personhood, but as personhood under the aspect of respect owed, then we are clear on the fact that dignity can no more be taken away or reduced than can personhood itself. I can't take away your dignity, I can only fail to give you the respect I owe you as a person.
Nor, for that matter, can I take away my own dignity; to be undignified is to act as though I weren't the person I am, but it doesn't actually make me not the person I am.
And I think here we can let "the person I am" have two meanings: the ontological meaning, according to which I am an individual substance of human nature; and the social meaning, according to which I am an individual situated in a certain place in my family, community, workplace, and so on.
(The point of being able to support both meanings is that it suggests such a concept of dignity is generally useful.)