A maid signifies a virgin. Hence, maiden name. Please see Samuel Johnson's Dictionary of the English language, the 1828 Webster's or any other reliable dictionary.
Perhaps Fr. Knox was simply trying to adopt a translation more natural to the English tongue as opposed to the highly useful and accurate, but at the same time slavishly and unnaturally Latinate translation of the pre-Challoner Douay-Rheims translation.
I had not thought about that Vladimir, but I think this is a case of "if its not broken don't fix it." The term virgin in Isaias 7 is clear and not a translation difficulty that can later be improved upon.
Honestly, when I first read Cuthbert's post, I was slightly alarmed at what at first seemed to be a deliberately ambiguous translation of, as you have said, a very clear term.
However, if Fr. Knox was doing what I think he was doing - that is, choosing a synonymous English term for the original Latin - then I, having worked on translation myself, can sympathize with his decision. The question is - if one is translating a work for speakers of the English tongue, why not find the idiomatic expression with the same meaning instead of merely putting in a cognate, albeit a common one? In fact, after making my previous post, I checked a dictionary just to be sure and, in fact, "virgin" was the first definition of "maiden" (after specificing that the subject was a girl of course)! It can't get much more clear than that.
For the great majority of laymen, the only versions of the Holy Scriptures that we read will be translations. It's really senseless to limit ourselves to one translation without giving other translations a chance, at least for comparison sake.
I have found this to be true with languages where the translations of Holy Scriptures were never standardized. It is great fun to compare the different nuances in translation of the different versions of Chinese Bible translations for example, or the Old Testament paraphrases done by the Jesuit missionaries into Vietnamese. What would this forum think of the translation of the book of Psalms into Classical Chinese by John Wu? At times they barely amount to paraphrases of the Douay (he didn't translate from the Vulgate), but yet they have been the source of great edification for the Chinese faithful and a great tool of evangelization - the same goes for his translation of the New Testament which is also liberally translated compared to the Douay translation of the Vulgate.
Hands down, the best English translation is the original pre-Challoner Douay. However, that shouldn't deter us from adding the Challoner Douay, the Knox, and other translations (so long as they are not blatantly Protestant) to our libraries!