There's been an interesting discussion on Nan Hawthorne's blog about historical accuracy in which the question of word usage has come up. Should writers of historical fiction try to "write forsoothly"? Or simply to avoid using any words that weren't current during the period in question? Or should they stick to modern language and usage? Or should they just say to heck with it and have a medieval English heroine speaking like a 1980's American teenager, as I've seen in some romance novels?
I won't get into the whole debate here, but I'll say that in my own novels, I've stuck to modern language and usage, though I try my best to avoid anachronisms (such as "sidetracked" or "railroaded") and modern slang. (I'm sure I haven't always succeeded, though.) Part of this is purely personal preference: Certes, I feel rather silly writing "certes." But I've also a nobler reason: reader sanity.
Take, for instance, a letter by the imprisoned Thomas Howard, Duke of Norfolk, to Henry VIII in 1546 when the duke, desperate to save his head by reminding the king of his past services, asked, "Who showed His Majesty of the words of my mother-in-law, for which she was attainted of misprision but only I?" Now, when the duke refers to his "mother-in-law," he is referring to Agnes, the Dowager Duchess of Norfolk, whom we would call his stepmother. So if I were writing a novel about the duke, I would be following contemporary usage if I had him refer to Agnes as his mother-in-law, but I would also be puzzling most readers. I could add a footnote or include a glossary, I suppose, but do I really want to take the readers out of the story by having them check a reference or flip back to the glossary? In my opinion, in a novel, it's easier for all concerned to simply let Norfolk refer to the old duchess as his stepmother and be done with it. (Probably after being incriminated by the duke, the duchess had her own way of referring to him, but medieval cursing is beyond the scope of this post.)
With regard to family relationships, one sees other contemporary usages that can confuse. In her 1480 will, for instance, Anne Neville, the Duchess of Buckingham, refers to her "daughter Richmond" and to her "sonne of Buckingham." The reader unversed in contemporary usage might well assume that the duchess is referring to her daughter and to her son; in fact, she is referring to her daughter-in-law Margaret Beaufort, the Countess of Richmond, and to her grandson Harry, Duke of Buckingham. Having the duchess stick to contemporary usage might well result in reader loss of hair, and I have no financial interest in the wig industry. Therefore, in my novel, I let the duchess use modern terms: "daughter-in-law" for Margaret and "grandson" for Harry. Just thank me when your beautician admires your full crop of hair.
Other differences in usage can lead to rather more amusing consequences. It's safe to say that when Jean Plaidy titled a novel Gay Lord Robert, she didn't anticipate the modern snickers that would ensue. I myself remember reading a novel by Betty Smith, set, I think, back in the 1940's, when the heroine boasted about "making love" to her husband in public. Now, the heroine was an extrovert, but not that much of an extrovert, so naturally this made my seventeen-year-old self perk up with excitement. Quickly, though, I discovered that what the heroine meant by "making love" was no more than what my high school principal termed "a display of affection." Still, though, the modern association the term has with "sexual intercourse" might make it risky to use in a historical novel.
Which brings me to yet another Duke of Norfolk, John Howard. In 1485, while preparing to resist the invasion of Henry Tudor, he wrote a letter to John Paston urging him to bring "seche company of tall men" to meet him. He signed the letter, "Yower lover." Now, no one would seriously suggest, I think, that John Howard and John Paston were "lovers" in the sense in which we would use the word; such fulsome language was not unusual in the fifteenth century, especially when one had a favor to ask. But in a novel, would I have a man use this language when addressing another man? Only if I did intend the reader to believe that they were lovers in the modern sense or only if I were prepared to add a great deal of explanatory context.
Did John Howard's appeal succeed, by the way? Nope. When the Battle of Bosworth was fought, John Paston was not there.