The English word enthusiasm derives from Greek entheos, “possessed by a god.” A person filled with enthusiasm was filled with a divine frenzy. An early meaning in English was “ poetic or prophetic frenzy.” An “enthusiastic preacher,” for example, was what a modern speaker might call a “charismatic speaker.”
From describing religious fervor, the use of enthusiasm extended to passionate feelings expressed in other areas, like politics.
In the rational 18th century, the word’s religious application acquired the negative connotation of irrational or delusional.
Samuel Johnson (1709-1784), himself a deeply religious man, was suspicious of people who claimed to be privy to the divine will. In his dictionary, he defined enthusiasm as “a vain confidence of divine favour or communication.” The meaning he attached to vain was not the one now current; by vain, he meant “unprofitable, pointless, futile.” In his essay on the poet Abraham Cowley (1616-1667), however, Johnson used the word with the meaning of “poetic inspiration”:
He [Cowley] was the first who imparted to English numbers the enthusiasm of the greater ode, and the gaiety of the less.–Lives of the Most Eminent English Poets.
The verb enthuse is documented from 1827. The OED etymological note calls it “an ignorant back-formation.” Merriam-Webster, as one might expect, is less judgmental:
Enthuse is apparently American in origin, although the earliest known example of its use occurs in a letter written in 1827 by a young Scotsman who spent about two years in the Pacific Northwest. It has been disapproved since about 1870. Current evidence shows it to be flourishing nonetheless on both sides of the Atlantic, especially in journalistic prose.
As a transitive verb, enthuse is used with the meaning “to kindle with enthusiasm”:
The liveliness of the dance enthused the audience.
Used intransitively, enthuse has the meaning “to grow enthusiastic; to go into ecstasies”:
Here I caught up with Parallel’s chairman David Ciclitura and group managing director Stewart Mison and listened as they enthused about the business potential offered by professional golf tournaments.
Flourishing or not, enthuse is a word that continues to raise hackles, so it’s best to avoid its use in a formal context.