Boorishness or loutishness is, in the context of virtue ethics, the vice of defect opposed to eutrapelia, the virtue concerned with proper amusements. Its recognition as a vice has a long history, going back to Aristotle, who recognized eutrapelia as one of the eleven virtues he discusses in the Nicomachean Ethics. Aristotle has relatively few comments on boorishness, however; besides identifying it as the vice of defect opposed to the virtue of eutrapelia and its opposing vice of excess, frivolousness or buffoonery, he also tells us that the boorish person both does not make jokes himself and objects to other people making them and contributes nothing to the amusement and relaxation of human life, despite the fact that these are necessary to it. The basic idea seems to be not that the boor is merely humorless, but that he is humorless in a way that imposes on others; he attacks people for making completely unobjectionable jokes. That is, it's not about funniness, nor about the ability to get funny jokes. Rather, it's about the refusal to allow space for joking at all. Beyond this, we get very little in Aristotle or anyone else, as far as I know, until Aquinas.
Aquinas's categorization of eutrapelia is somewhat complex. It is associated with temperance as its principal virtue. One of the potential parts of temperance is modestia, which might perhaps be translated as 'appropriateness'. Potential parts are associate virtues, having a great deal in common with their principal virtue, but distinct from it in some circumstance. Thus modestia is a kind of temperance-in-a-broad-sense. It has what appears to be three subjective parts or species: appropriateness in serious changes of body, appropriateness in playful changes of body, and appropriateness in outward apparel. Eutrapelia is the second. As in Aristotle, boorishness is the vice of defect, and, in fact, Aquinas usually just calls it the vice of defect in play, rather than any other name. Aquinas's eutrapelia is arguably somewhat broader than Aristotle's, being something like moderate playfulness rather than just wittiness, so his corresponding vices are broader, as well. The reasoning behind the need to identify defect in play as a vice is that it is against reason to make oneself burdensome to other people by giving them no delight and impeding their delight in other things. This is precisely what one deficient in play does to other people however, since he is not playful and objects to the moderate play of others.
However, because play exists not for its own sake but for the sake of delight and rest, and because delight and rest are themselves for the sake of activity, it follows that deficiency in play is less vicious than excessiveness in play, or, in other words, boorishness is less bad than frivolousness. The boorish person at least genuinely takes serious things seriously, even if he badly errs in trying to make everything serious; but, since play is to make the serious activity of human life possible, the frivolous person, in trying to take everything playfully, fails to treat serious things as they ought and fails also to treat playful things as they ought.