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On December 10th, 2000, a definition for "Yiff" was added to the website Everything2 by user Speck, going into detail about its origins as a greeting in a roleplaying language. The following year, Everything2 user wrote:
Speaking as the person who, in ancient days of yore, played the terrifyingly cute character littlefox (all lowercase!), I have to say that the accounts given above aren't quite correct.
The original foxish language definition was as follows:
"The basic sounds are Yerf, Yip, Yarf, Yaff, Yiff, Growf, and Growlf. The general order of sounds from positive (happy, good, greeting) through neutral to negative (mad, unhappy, no), are: Yiff, Yip, Yerf (positive), Yaff, Yarf (neutral), Growf, and Growlf (negative). Murphle (a sound popularized by some other characters at the time) is a sound of contentment. The basic sound, if modified by an -le suffix, is less absolute, less emphatic. ie: Yiffle is less emphatic than Yiff. There are a couple of words that have somewhat general meaning, though the entire language is rather overloaded. Yerf's are generally used in greetings, though exuberant greetings are Yiffs. Growf means no, and a Yiff means a yes. (an example of the overloading of the language.) There is one term that got defined by someone who shall remain nameless: Yipp is generally considered not polite, as it is basically a sexual proposition."
Later on, some wolves redefined Yiff to more or less take the place of Yipp, as a generic term for sex. It soon became a noun, a verb, an adjective, and about as many other parts of speech as smurf. Oh, and yes, I'm aware that foxes have never made half the sounds listed above. They were cute sounding at the time (about ten years ago). That was reason enough.
On January 17th, 2003, the first definition for the term was added to Urban Dictionary defining it as "To make love to a plush animal" and "A plush animal that has been spooged on." It obtained 117 thumbs up and 483 down. Later, on August 20th, 2003, another definition was submitted, giving it multiple definitions. It obtained 3615 thumbs up and 675 down. Later, on March 16th, 2004, a page for the word was added to WikiFur. A page for it was added to wikitionary on September 6th, 2007.
Yiff in Hell
"Yiff in Hell" is a phrase used by those that oppose the furry fandom, directly aimed towards furries as a message to take the activities commonly associated with yiffing elsewhere. While only a couple of image macros with the exact phrase "Yiff in hell" can be found, a search for pages containing the phrase amount in excess of 600,000 and the longer, "Yiff in hell, furfags" makes up 1/6th of that.
On 4chan, "Yiff in Hell" is typically found in Furry threads, used as a protest towards furries in an attempt to lower the amount of furry content posted on the site. While it has been seen spammed throughout threads in great numbers, certain self-identified furries have offered rebuttals stating that the argument doesn't make sense.