Put here because I think it’s a good definition, and as a reference for myself:
I recently read a good definition of “trustworthiness.” We decide that someone is worthy of our trust when we believe that they will take our best interests into account. That doesn’t mean that they’ll do what we want. It means that they’ll consider our best interests and will take them seriously.
via Wil Wheaton (I think on his Tumblr; I can’t find the post now.)