It’s an adjective and it means not having or lacking enthusiasm. If someone is unenthusiastic about something it means they are feeling indifferent, apathetic, passive, phlegmatic, lukewarm, listless, languid, lethargic, emotionless, perfunctory about it.
I’m unenthusiastic about having Malcolm on the team.
by AKACroatalin March 24, 2017
It is an opposite meaning of enthusiastic. A type of scepticism. If someone doesn't believe in success.
John was unenthusiastic but gave it a chance for win
by iGame_again June 27, 2016