Deadly Nightshade - Belladonna - Why is it called that?

The usual explanation one hears for the name of the deadly nightshade as "belladonna" is that it was used as a cosmetic eyedrop to make women beautiful. Another popular claim is that, because of its alleged use by witches, that it's a euphemism for a witch (compare the word beldam) or perhaps a euphemism for the ancient goddess worshipped by the witches. 

No one ever has produced written evidence to back up either of these claims.

For the eyedrop theory, one would like to see a historical recipe for a purely cosmetic eyedrop made from the substance, and evidence of it in widespread use. All one finds are medical recipes used to treat diseases of the eye. The very idea of cosmetic eyedrops was rare in the era when the belladonna word appears, partly because it was hard to maintain purity of the ingredients to make a consistently eye-brightening formula. Even into the 19th century, eating a spoonful of whiskey and sugar was the kind of semi-superstitious beauty treatment followed for eye beautification, due to lack of better alternatives. It should be noted that there's no record of this proposed eyedrop etymology till the 18th century, hundreds of years after the term came into use.

The witch idea also lacks anything to back it up. For this, one would expect the herb to have other streghe related names, or mentions of its being used in witchcraft in the historical herbal texts if it was really so well known for this purpose. (Don't suppose the literate herbalists would have hesitated to admit its use in witchcraft -- they're happy to say if something was used by Circe or a famous poisoner, not to mention the legitimate medical uses in this era aren't much different than magical use. Moreover, this is an herb whose primary merit is narcotic. Mentioning it at all is admitting dubious stuff can be done with it.) It's also notable that the belladonna name was regionally specific to Venice, which is hardly a place one thinks of as the domain of simple country folk preserving thousand year old pagan traditions unhindered by modern (16th century) progress.

The best explanation for this plant's curious name is that it's a folk etymology. These often happen when an unfamiliar word gets rendered by the native speakers of the language into something that sounds more familiar. For example, sovereign is from Old French souverain and related to the Italian word soprano. The added G in the English word is from folk etymology associating it with the unrelated word reign, like a sovereign does. Penthouse is another folk etymology, from Old French pentis, originally Latin appendicium, in a specific meaning of an attached room or shed onto another building. The French -is ending came to be taken as the English word house.

Here is what we know for sure about belladonna: its first use by that name in English is in Gerard's herbal, 1597. He almost certainly got the term from the writings of Matthioli, perhaps in Latin translation or perhaps he read it in the original Italian. Even in the Latin translations, "bella donna" is used as Matthioli's word for what is usually termed in period texts solatrum or solanum or morella in Latin. Matthioli mentions that in his native Venice it's commonly called herba bella donna or solatro maggiore. The work in question is a commentary on the work of the ancient Roman doctor Dioscorides, so he is mostly just concerned with trying to give his readers modern names for the plants Dioscorides called strychnos, alikakabos, and so forth. He seems particularly concerned with clarifying that it's not the same plant as mandrake, which was presumably a common mistake, in his time, that needed addressing.

(Something important to think about when looking into historical plant names, is that the modern scientific classification system didn't exist yet. How these folks were defining types of nightshade doesn't always gel with how we decide this nowadays. In fact, the "deadly nightshade" was scientifically classed as a totally different plant from other nightshades, being given the genus atropa, whereas garden nightshade is classed in the solanum genus. By extension, the "nightshade family" now includes edibles such as potato, eggplant and chile pepper.)

Other have tried to decipher the mysteries of how this plant acquired the folk name belladonna. Even 20 years ago, this would have been a nightmare to do. You would have had to figure out by yourself what kind of period texts might have information you wanted, write to various libraries for permission to view their antique books or send you photocopies of the pages, then read them all cover to cover to see if indeed anything did have what information you wanted, which 99% of the time it probably wouldn't. Now, we have searchable online databases of texts that will only show texts that have the subject you search for. It takes you straight to Matthioli's entry about bella donna instead of having to read the whole thing. It also shows you if anyone else in the era was writing about bella donna in a plant sense, and the answer appears to be no, since nothing else turns up in the searches.

Since bella donna seemed to be a regional term with no equivalent in other languages till after Matthioli started getting spread all over the place, we have to look to before and see if anything had similar names. Now, there are some scholarly theories floating about, often 100 years old or more, about what the bella donna plant might be named for. The plant called bladona has been proposed, but it's not a nightshade -- in fact it's usually identified as mullein. An unattested Proto-Indo-European word for a "crazy plant" has been proposed, but of course isn't able to be proved (the evidence for the existence is several other narcotic plants with a bel element in their name, but this might simply be from the PIE for leaf, *bhel-.) To avoid the pitfalls of these theories, one needs something attested and which is some kind of nightshade or at least used in a comparable way.

Since one has no idea what this plant might be, one has to just start reading herbals and looking for words. Since the word is of Italian origin, Italian herbals seemed the place to go. 

Indeed, in the 14th century herbals of Simon of Genoa and Matthaeus Silvaticus have a name written belhulidus, belulidus, belbulidus or besulidus, and given as an alternative name for the kind of nightshade called alkekengi. The inconsistent spellings suggest it's not a Latin word, but rather an attempt at Latinization. It also turns up here and there in other medical books, always of Italian origin. Already one can imagine how belhulidus can transform into bella donna by Italian folk etymology, but it would be nice to have an actual link. That was found in the work of Luigi Anguillara, a contemporary of Matthioli who gave us biasola as a name for the morella. Compare to the besulidus. Now, the names are getting thrown around to different plants a bit, but no worse than the word nightshade itself does. Illustrating this article is a 15th century Italian manuscript page that labels the alkekengi-besulidus-belulidus as also the solatrum major (solatro maggiore of Matthioli's "vulgar Latin" i.e. Italian.)

This seems to be the best answer so far, and certainly better than other theories that don't have any evidence for them at all.