This Latin expression is composed of virtus, virtue, and dormitiva, feminine of dormitivus, dormitive.

It first appeared in the following lines in dog Latin of Le Malade imaginaire (The Imaginary Invalid – 1673), in which the French playwright Molière (Jean-Baptiste Poquelin – 1622-73) satirised the circular explanations sometimes used in early medicine:

(original edition)
BACHELIERVUS.
Mihi à docto Doctore
Domandatur causam & rationem, quare
Opium facit dormire ?
A quoy respondeo,
Quia est in eo
Virtus dormitiua,
Cuius est natura
Sensus assoupire.
     translation:
I am asked by the learned doctor the cause and reason why opium causes sleep. To which I reply, because it has a dormitive property, whose nature is to lull the senses to sleep.

In English therefore, one of the meanings of virtus dormitiva is an explanation which merely restates in different words the very thing which is to be explained. This is what is alluded to in an article titled On the origin and formation of tubercles in the lungs, by J. L. C. Schronder van der Kolk, translated from the Dutch by William D. Moore and published in The Dublin Hospital Gazette of 15^th February 1858; the author wrote the following about an explanation given by the Bohemian physician and pathologist Carl von Rokitansky (1804-78):

I must confess, with all the respect which I feel for the indisputable merits of Rokitansky, that this mode of explanation always brings vividly to my mind Molière’s saying about the virtus dormitiva in opium.

The Latin expression also means a sleep-inducing property or effect, as in this passage from Music as a hypnotic, published in The St. James’s Gazette (London) of 9^th March 1896:

There is ample evidence that there is in music what Molière’s immortal Bachelor would call a virtus dormitiva.

The following is from Bedside books, published in The Falkirk Herald (Stirlingshire, Scotland) of 26^th July 1905:

The bedside book must not be heavy, either in the figurative or in the literal sense. It should also keep open without calling for special vigilance on the part of the reader. Some of our popular magazines, which otherwise would be boons to the recumbent reader, carry such a load of advertisements, and are withal so clumsily stitched that the literary matter cannot be read without an uncomfortable amount of muscular exertion. When reading is deliberately used as a means of wooing sleep, graver forms of literature may be chosen. On the whole, we think poetry of the sublime order the best for the purpose. We have ourselves found Milton especially useful, but almost any epic may be recommended with tolerable confidence. Sir Henry Holland found the sonnet, in whatever language written, the most effective soporific. Perhaps to obtain the full effect of the virtus dormitiva that lies in serious poetry it should be read to us in a droning monotone; such service, however, is for most of us an unattainable luxury.