Apparently its roots are in some Teutonic or Germanic word meaning “noble,” and a number of historical (male) figures in early Britain had “Ethel” in their name (like Ethelred and Ethelbert).
At what point did it become a woman’s name? I’ve never known any men named *“Ethel.” (Admittedly, other than my own grandmother, I’ve never know any women named “Ethel,” either. It doesn’t seem to be a fashionable name anymore.)
*Notwithstanding Chief Francis Ethelbert Sharkey from Irwin Allen’s “Voyage to the Bottom of the Sea” series.
The rulers you mention are often spelled “Aethelred”, “Aethelwulf” and so on. The spellings may have become modernized in a later age. The meaning remains the same. If you search on “Aethel”, you will find a lot of 19th and 20th century “Ethel”'s being deliberately archaic, but I do find occasional references in genealogies indicating that “Aethel” was a woman’s name in the 12th century.
“Ethel” was a popular name in the 19th and earlier 20th century. I suspect it went out of favor when it started sounding like you were taking the kid’s name off the gas pump. Apparently, Ethel Merman was not the right type of celebrity to get a lot of kids named for her, either.
For names that are on their way to becoming girl’s names after having historically been boy’s names, consider Jordan, Leslie, and, my personal favorite, Kelly.
But another good example is Jody. This name doesn’t really
seem in any way feminine, if you look at it objectively,
yet it’s always struck me as a girl’s name. This is probably because my mother had an old friend, during the days of my oldest extant memories, named Jody.
A few years ago Jordan was one of the most popular names for newborn girls, but not even on the charts for newborn boys. Odds are that if you see a child in school named Jordan, it’ll be a girl.
Jody was Billy Crystal’s character on Soap. I have to wonder if that didn’t color the perceptions of at least one generation.
I’m not sure if there’s anything specific that makes a name “feminine”, at least in English.