The present participle of 'enthuse': to show or express great enthusiasm about something.
From enthuse (back-formation from enthusiasm in the 1800s) + -ing. Rather than deriving from the Greek like enthusiasm, 'enthuse' was created backwards from an existing noun.
Enthuse is actually a 'back-formation'—people were so enthusiastic about enthusiasm that they invented a verb from the noun, working backwards! Linguists call this creative language-making, and it happens all the time in English.
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