Reverse of the STUNNING SPELL (see).
Spell used to revive a person who has been hit by a Stunner.
See Edits and Changes to the Text – GF; this spell has been officially renamed by JKR from its original name. Some versions list this spell as “Enervate,” which changes the etymology quite a bit. In fact, if the word was really Enervate, the Latin origins would have exactly the opposite meaning from what it meant as Ennervate.
References from the canon
"en-" Old French from "in-" L. cause to be + "nerves" Eng. c.1603 strength, from "nervus" L. nerve