As may be gathered from the words of Dionysius (Div. Nom. iv), beauty or comeliness results from the concurrence of clarity and due proportion. For he states that God is said to be beautiful, as being "the cause of the harmony and clarity of the universe." Hence the beauty of the body consists in a man having his bodily limbs well proportioned, together with a certain clarity of color. On like manner spiritual beauty consists in a man's conduct or actions being well proportioned in respect of the spiritual clarity of reason. Now this is what is meant by honesty, which we have stated (1) to be the same as virtue; and it is virtue that moderates according to reason all that is connected with man. Wherefore "honesty is the same as spiritual beauty." Hence Augustine says (Q83, qu. 30): "By honesty I mean intelligible beauty, which we properly designate as spiritual," and further on he adds that "many things are beautiful to the eye, which it would be hardly proper to call honest."
The word translated here (very unsatisfactorily) as 'honesty', honestas, actually means the opposite of the shameful, the sort of thing one approves or honors when one sees it, and so would be better translated by words related to 'honor'. Honestas is being honorable. Thus Aquinas's argument is that honorableness is a spiritual beauty recognized by the mind. The term can just be a synonym for 'virtue', but it is especially associated with the virtue of temperance, and can be used to indicate one of the fundamental components of temperance: being the kind of person to treat spiritual and intelligible beauty as more important than physical pleasures.
