being worthy of good things, robs himself of what he deserves, and [has] something bad about him from the fact that he does not think himself worthy of good things, and seems also not to know himself; else he would have desired the things he was worthy of, since these were good.
Aristotle sees magnanimity as a sort of justice to oneself, so the pusillanimous is likewise a sort of failing to do justice to oneself. When Cicero discusses magnanimity in De Officiis, he shifts the account so that magnanimity is associated primarily with fortitude rather than justice. The magnanimous man is not so much honoring himself as he is making himself honorable. He does this, first, by a contempt for outward incidentals, in which he keeps his mind on the right and rises above the passions; and second, by seeking out difficult challenges that contribute to the public good. The mark of the small-minded or pusillanimous man, on the other hand, is the kind of passion that leads one to evade this kind of greatness-seeking -- the most obviously problematic of these being a love of wealth, but fear, lust for fame, sensuality, and the like may be motivators of the small-souled, as well.
Aquinas's account of magnanimity blends Aristotle and Cicero, so it is unsurprising that his account of the vice of pusillanimity (ST 2-2.133) shows the effect of this. Like Aristotle, Aquinas takes magnanimity to concern honor for one's own virtue, but, like Cicero, he takes this to be associated with fortitude, because our regard for our own virtue is more concerned with what we will do than it is with what we have done. We have a natural inclination to accomplish things suitable to our capabilities; this inclination is part of who we are as rational beings in the first place, which means that failing to fulfill it is to do something morally wrong. Magnanimity is the virtue concerned with this kind of accomplishment; the two vices are presumption, in which we strive to do more than our capability can reasonably be considered to allow, and pusillanimity, in which we refuse to achieve what is appropriate to our basic capabilities. He gives as an example the parable of the talents, in which the fearful servant buries the talent he has received instead of daring to trade it so that it will multiply.
Pusillanimity, therefore, is the habitual disposition of burying one's talents, in which we fail to strive for a greatness commensurate with our gifts, whether those be gifts of nature, or gifts of education, or gifts of fortune. It is not a kind of humility, although it might superficially look like it; it is often even motivated by pride, because one way in which we can be proud is by clinging to our own opinions simply because they are ours, and not letting ourselves be corrected. This can happen regardless of what our opinions are, and so we can, out of pride, refuse to be corrected in our false opinion of our own incompetence. It is a truly remarkable feature of human capacity for obstinate self-importance that we can value ourselves too little by valuing our own opinions too much. Pusillanimity can also arise in other ways, however; for instance, we can shrink from greatness due to laziness in considering our capabilities, or because we have in some way become disheartened by other passions.
In the post-medieval era, Spinoza is notable for considering, at least to some extent, the moral problems of pusillanimity, in scattered discussions throughout the Ethics; a good example is in Part IV, Proposition XXIX, in which he characterizes it as thinking too little of one's self due to pain. It is a sort of humility -- which he regards not as a virtue but as a passion to be overcome -- taken to extremes. But he also regards it as rare; in fact, he thinks that the people who usually come across as so self-denigrating are in reality dissimulating, and that this elaborate self-denigration is often a cover for a pettily envious and ambitious mind.