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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."
de Jedimintrick 25 octobre 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.
de Blue Jack 18 octobre 2009