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Connected with Anthropomorphology, yiffed is the past participle of yiff. (That means that it is the past tense of yiff) When two or more anthros have mated, you say, they have yiffed

Yiff, Yiffing, Yiffed. In that order
When we finally got home we yiffed all night.
by Player May 06, 2005
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Nov 27 Word of the Day
A stupid person; it refers to the lack of surface area on an individual's brain. The general thought is that the more surface area (wrinkles, creases, etc.) a brain has, the smarter the person is. Conversely, a person with a smooth brain (no wrinkles) has less surface area and would therefore be stupid.
That fucking smooth brain put his shirt on backwards again...

That smooth brain is dumber than a pile of shit.
by Tip Tank May 14, 2011
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