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1.
The act of feigning enthusiasm.

Usually sarcastic, but can sometimes be a means of gratifying a dull anecdote.
"I had to use my unthusiasm to get rid of that guy."

"Hey man, no need to use unthusiasm on me."
by Jedimintrick October 25, 2006
 
2.
Lack of enthusiasm.
Either sarcastic enthusiasm, or from boredom.
1) Woop. Wa-hey. Yeah. This is fun.
2) Nice going with the unthusiasm there.
by Blue Jack October 18, 2009