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15.
To throw up. To feel queasy or sick.
"God, that is making me yiffy."

"I think I'm going to yiff."

"I just yiffed in the sink."
by P. Katto June 03, 2003
 
16.
v. To make love to a plush animal.

adj. "Yiffy" A plush animal that has been spooged on.
I think KickFag gets yiffy with that plush tiger in his room.
by Mario Rogic January 17, 2003
 
17.
v.

1.) Furry code word for "violently rape." Usually used in reference to the anal ("tailhole") orifice of a male animal they intend to penetrate with their sad excuse for a penis.
2.) To dress up in a fursuit, find another furry dressed likewise, and hump them while in the fursuit. Sole form of furry sexual gratification not involving the need to "paw off".
1.) "Hey, let's go yiff that sheep!" said GoldenMan323 to his furry friends. Unfortunately, Farmer Bob disagreed with the idea.

2.) Watching furries yiff is like watching a Cheney vs. Rove boxing match. Nobody wins, and everybody who watches it is gonna lose.
by Lenin's Bastard Son September 11, 2004
 
18.
It stands for, Yucky Idiotic Furry Fuckers. Y.I.F.F.
All furries are yiff.
by Ryan Satter March 21, 2005