enrage v.

word-forming element meaning "in; into," from French and Old French en-, from Latin in- "in, into" (from PIE root *en "in"). Typically assimilated before -p-, -b-, -m-, -l-, and -r-. Latin in- became en- in French, Spanish, Portuguese, but remained in- in Italian.

Also used with native and imported elements to form verbs from nouns and adjectives, with a sense "put in or on" (encircle), also "cause to be, make into" (endear), and used as an intensive (enclose). Spelling variants in French that were brought over into Middle English account for parallels such as ensure/insure, and most en- words in English had at one time or another a variant in in-, and vice versa.

c. 1300, "madness, insanity; fit of frenzy; rashness, foolhardiness, intense or violent emotion, anger, wrath; fierceness in battle; violence" (of storms, fire, etc.), from Old French rage, raige "spirit, passion, rage, fury, madness" (11c.), from Medieval Latin rabia, from Latin rabies "madness, rage, fury," related to rabere "be mad, rave" (compare rabies, which originally had this sense). This is said by some sources to be from PIE *rebh- "violent, impetuous" (source also of Old English rabbian "to rage"), but de Vaan finds this uncertain and sees no convincing etymology.

Similarly, Welsh (cynddaredd) and Breton (kounnar) words for "rage, fury" originally meant "hydrophobia" and are compounds based on the word for "dog" (Welsh ci, plural cwn; Breton ki).

It is attested from late 14c. in the sense of "fit of carnal lust or sexual desire." In 15c.-16c. it also could mean "rabies." Other Middle English senses, now obsolete, include "come to a boil; grieve, mourn, lament; flirt, make love." The rage "fashion, vogue" dates from 1785.

"A normally calm person who has become suddenly enraged or violent" [OED], 1932, from French mouton enragé, literally "angry sheep." See mutton + enrage.