For several centuries, the primary definition of enervate was to weaken or to sap of energy or will. In current usage, however, it is often used as a synonym of energize—almost the opposite of its older sense. Many people understandably consider the older sense the only correct one, and careful writers and editors continue to keep it alive, but this might not be enough to stop the spread of the word in its newer sense.
Enervate comes from the Latin enervare, meaning to cut the sinews of or to weaken.1 The word entered English around the late 16th century and was mostly figurative (i.e., not referring to the actual cutting of sinews) right from the start.2
It’s difficult to trace exactly how or when the newer sense entered the language. In our own searches of historical texts we find no examples of envervate used in the newer way from before the late 1990s, but this doesn’t mean there are no earlier examples out there. In any case, it was not until this century that the second sense became so common that examples are easy to find without much digging. In fact, the newer sense is more common than the older one in the types of web-searchable writing that are typically not well-edited.
It is worth noting that enervate‘s near homophone innervate, which is used mainly in biology, is roughly synonymous with energize if we stretch it into figurative use. Perhaps this has had some influence on the development of enervate‘s newer use.
Since the 17th century, enervate has been primarily a synonym of weaken, as in these examples:
And therefore lest his Impostures and Deceits should be discovered by the light of the Word, he lets himselfe by all meanes to overthrow the Scriptures, to enervate and weaken their Authority. [The Plain mans Senses Exercised to Discern both Good and Evil, William Lyford (1655)]
The rational faculties are so enervated, as to be disordered on every trifling occasion; the patient comes to be troublesome to others, as well as to himself. [Attempts to Revive Antient Medial Doctrines, Alexander Sutherland (1763)]
To accomplish this design, he studies to protect his industrious subjects, and to moderate the violence without enervating the valour of his soldiers. [The history of the decline and fall of the Roman empire, Edward Gibbon (1837)]
Endless morasses of reeds enfolded us, in whose miry depths the foot sinks even in the dry weather, in which the sultry heat enervates us. [In wildest Africa, Carl Georg Schillings (1907)]
That goal seemed to invigorate the United States and enervate El Salvador, which had dominated play for more than an hour. [New York Times (2009)]
But today, enervate is commonly used as a synonym of energize—for example:
Drohan, her initial shift from near-catatonia to mega-smiling politesse reaching hilariously enervated levels — “Great! Great-great!” — is a comic find. [Los Angeles Times]
It’s an enervating and invigorating aspect of contemporary society that the apparatus of luxury, once denied by status and birth, is more widely available. [TVNZ]
Such examples are difficult to find in edited writing, but they are abundant in web searches covering internet forums and blog comments (which we don’t like to use for examples).