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1.
(1) This word is to be used in a time of frustration. It can be used in place of damnit when things are not going your way.

(2) It is another word for a retard or idiot.
*Cop pulls you over. "Derfner!"

"That kid is such a derfner!"

"Our president is a derfner, at best."
by TheRealBossDizzle May 06, 2010