Using the language rules Tolkein would have used, for example the plural a dwarf being dwarrow. I’m assuming it is not like Latin and simply adding an a but more complex.
Rada is what I’d call her.
Or Radagastia
Or more creatively Radagastrix
Radagath, or Radaga.
My guess after 2minutes of googling and finding this Page
Radagast is a puzzling name. It’s not at all clear what language it is supposed to be in: whether it’s a literal word in an untranslated language like Quenya, Sindarin or Adunaic, or whether it’s from a “translated” language like the Common Speech (translated into English) or Rohirric (translated into Anglo-Saxon). (However it is certainly not Adunaic as yendis suggests, since The Hobbit was written more than 10 years before Tolkien conceived Adunaic.)
In The History of The Hobbit, John Rateliff discusses the possibility that it could be Sindarin, but there are few plausible Sindarin roots that would produce such a name. More likely, it could be a name from a Mannish language translated into a real world language. This is supported by a late note that Tolkien wrote: “Radagast: a name of Mannish (Anduin vale) origin - but not now clearly interpretable”. So what is the real world source of the name? Rateliff suggests either Old English, a Slavic language, or Gothic.
Old English fits well with the general linguistic setting: other languages of the Anduin vale are translated into Old English. And in Old English “rad” means “road” and “gast” means “spirit” or “angel”, so “spirit of the road” seems a plausible meaning. But there are other possibilities.
Slavic is a surprisingly strong contender despite the lack of other Slavic-derived words in Tolkien’s work. The character Beorn (who in The Hobbit says that he know Radagast ) was originally named Medwed, certainly a Slavic name. And there was a Slavic god (or possibly town) named “Radegast”. (There is also a Czech beer named after the god.)
But Gothic is the source Rateliff prefers. There was a Gothic king named Radagaisus or Rhadagast. Tolkien was keenly interested in the Gothic language and used Gothic names for other characters from the Anduin vale, notably for most of the kings of the Rohirrim’s ancestors.
So we should probably look to Gothic to determine a feminine form. Gothic is poorly attested, mostly surviving in one single document (which is probably one reason why Tolkien was so attracted to it; he liked that kind of linguistic mystery). But I don’t know enough about the language to say how feminine names were constructed, or even if there is enough surviving context to answer the question at all.
[Moderating]
Tolkien’s linguistics are well-enough documented that there’s a chance that this question could actually be factual… but it’s clearly, unambiguously a question about literature, and so I think that this thread is a better fit for Cafe Society. Moving.
I’ve done a little poking around the glossary for the Codex Argenteus, the primary Gothic text. It’s part of a translation of the Bible.
The name “Mary” is rendered “Marja” (in the nominative case) and “Salome” is rendered as “Salome”. I don’t see any other female names. That isn’t much to go on. However I note that among the male names, there isn’t a consistent pattern to the name ending. So we have “Agustau” (Augustus), “Daweidis” (David), “Filippus” (Philip), “Helias” (Elias), “Iakobu” (Jacob), “Iosef” (Joseph), “Paitrus” (Peter) and “Seimonaus” (Simon). So there apparently isn’t a rigid rule for constructing either masculine names nor feminine names; they mostly seem to be close to the Latin names.
So I dunno, seems like you’re free to do pretty much whatever you want. Beck’s “Radagastia” seems as good as anything. Or “Radagasia” or “Radagasa”. Or substitute a “j” for the “i” to make it look a little more exotic.
Not sure if it helps, but there was the slavic god Radegast (god) - Wikipedia. Very similar and I suspect the Professor was aware of him.
Some translations has it as glad guest, others as forest god.
I do recall JRRT saying that Frodo’s actual name was Froda, derived from an old english name ‘Frod’, meaning ‘wise by experience’. The ‘a’ ending is masculine in old english, but JRRT changed it to Frodo to conform with his practice of ending male hobbit names with an ‘o’.
JRRT also reported that Frodo’s name in Westron was ‘Maura’ in “Peoples of Middle-earth”.
Anyway, probably not a good assumption to think that Radagasa must be feminine.
Yes, in Appendix F to LOTR, Tolkien said that masculine “Hobbit names” end in “a” and feminine names end in “o” or “e”.
It’s not clear whether the o/e/u rule applies to Westron names in general or just Hobbit names. Whatever Radagast is, it’s not a Hobbit name, it’s probably not native Westron, and it’s likely not translated Westron (English). Above I present Rateliff’s argument that it’s Gothic, in which case it should conform to whatever rules apply to Gothic names, although given the evidence available I’m not there are such rules.
Asking for the feminine form of Radagast may be like asking for the feminine form of Arthur or Matthew. There just isn’t one, although you can of course make up a name that resembles Radagast and sounds feminine in some language. Ideally it would sound feminine in its native language (Gothic), but if that’s not possible second best would be sounding feminine in ones own language (ie. English).