Content warning for the obvious

I happen to be named neither Desmond nor Seward, so I can't promise that the following is what he meant. Nevertheless: while "sluttish" (or slutty) is a terrible way to say it and he shouldn't have gone there, the concept it conveys--in our standard usage--is not wrong. That's because a "slut" isn't a promiscuous woman, or a woman who acts in a sexualized manner in public. It's primarily a woman who is judged to be too sexual. And for the royal bride in question--French queen Isabeau of Bavaria--"judged to be" is exactly what we're talking about.

Isabeau of Bavaria (1371-1435) was

a frivolous spendthrift surrounded by a flock of scandalous ladies

who was guilty of

adultery, cupidity, neglect of her children, and political incompetence

so in conclusion, she was

disruptive, conniving, malicious, and publicly dangerous

to borrow some summaries from the scholarship of Tracy Adams (the leading scholar on Isabeau). You could also look to the question I've answered on AskHistorians about Isabeau: Did she popularize nipple piercings in late 14th century France?

But that user will go on to add, "The writer cites Eduard Fuchs by way of Hans Peter Duerr's book Dreamtime, but with a lot of caveats. ("The paragraph was patched together by Mr. Duerr using three sources, some written over 60 years apart and in different languages.** Until more research is done, one can only deduce that** the fashion of the time led to a trend of piercing nipples at some unspecified later time, perhaps months or even years later.")

And as I discuss in that answer, Duerr's sources do not say what he claims they do, either.

In fact, what Adams offers up there is a summary of Isabeau's reputation, which survives through today. Since the 1970s, scholars have shown Isabeau to be a sexually typical, politically agile queen and regent who made a few enemies along the way. (Little things like switching sides in a bloody family/territorial feud will do that to you.)

The origin of myth, scholars argue, goes back to the Orleanist-Burgundy feud and one of its crisis points, 1405-07. Duke Philip of Burgundy had been de facto co-ruler of France with the Duke of Orleans until his death in 1404. Philip's son Jean thought he should get the same role; Louis disagreed. He and Isabeau, as wife of an indigent king pushed to the side, locked Jean out of power and money.

Jean decided to take them both down in gender-appropriate methods. He assassinated Louis, and he assassinated Isabeau's reputation.

At least, that's what we learn from Michel Pintoin, a chronicler from the abbey of St.-Denis. He notes four different ways that everyone was saying Isabeau was terrible. Most relevant to current purposes, Pintoin cites another author's words: "Venus occupies the throne in your court...Certainly drunkenness and debauchery follow her, turning night into day, with continual dissolute dancing." Also, Isabeau is de facto ruling the kingdom when she shouldn't be, and also, she's a bad mother.

Supporting Pintoin's contentions of contemporary animosity towards Isabeau, a political pamphlet from 1406 likewise tarnishes her reputation.

That is, of course, when one points out that Pintoin was very much on the Burgundian side, and the pamphlet is likewise tilted that way. Modern scholars concluded rather decisively that the root of Isabeau's slut-shaming was Jean's propaganda and its influence over contemporary perception of her.

Intriguingly, though, Adams and Glenn Rechtschaffen have recently gone even further. They argue that Pintoin and the pamphleteer are not, actually, reflecting contemporary opinion of Isabeau or even shaping it. For example, there are a ton of other chronicles of the period (or rather, "a ton" in medieval terms). Not only do they not slut-shame Isabeau, but they don't even report that anyone else was doing so. Since reputation or fama in the Middle Ages was a crucial social structure to the point that it could be legally admissible evidence in court, Adams and Rentschaffen argue that negative sexual talk about Isabeau couldn't have been very widespread during her lifetime.

And yet, like so many other queens and powerful women seen as "too powerful" at the time or in retrospect, we wind up with Internet articles that paint her as overly sexual and scholarship that labels her beautiful and a slut.

So when Seward calls Isabeau "sluttish," he's not just being a scholar. He's part of the problem.