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1.
1 —used to express actual or feigned ignorance or stupidity
2 —used derisively to indicate that something just stated is all too obvious or self-evident
"Twilight is bad?"
"LE FER DER! How did you not know that?"

"Wait, so you're telling me all women have vaginas?"
"Omigod, LE FER DER!"
by Charelz Dee December 01, 2009