"Prowess," when referring to medieval virtues, has to do with strength and skill in battle. Cathyn Fitzgerald wrote " [P]rowess is. . .nothing more than physical skill. In our Society, this skill translates to winning swordfights. Prowess without Chivalry is cheating, Prowess without Honor is murder, Prowess without Modesty is braggadocio. Prowess without the other virtues is no virtue at all."
Within the SCA, this all makes sense. Historically, it might make less sense, in a society in which "cheating" wasn't such an issue. It's interesting to think about the differences, and the value placed on tempering prowess with virtue.
Sir Garick von Kopke wrote:
Prowess is an oft taken for granted or forgotten virtue when discussing the philosophy of Chivalry, but it must remain pre-eminent…. Prowess at arms is one thing that your character cannot bring you, nor can you will it to be. Your character and will, however, can bring you to the persistence needed to learn the skills of arms.
Prowess: To seek excellence in all endeavors expected of a knight, martial and otherwise, seeking strength to be used in the service of justice, rather than in personal aggrandizement.
Brian R. Price
From Steven Muhlberger's translation of a 14th century text:
And it should be noticed and seen in the deeds of this valiant man, how without doubt just as the proverb says, that it is by works, and not at all by words, that the spirits of valiant, gallant men show themselves, for there is no doubt that the man who has the spirit and desire to reach and attain honor never thinks but to consider how and in what way he ought to do such deeds, that he should deserve being called valiant; nor ever should it seem to him that he has done enough, whatever good thing he should do, to acquire praise for his valour and prowess.
Way back when ( Indo-European days), the basic words for goodness and uprightness and first of rank split out to become various different concepts and words, and "prowess" and "probity" are related at that level. It has to do with being proven to be superior.