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1.
1) (noun) Sound supposedly made by mating arctic foxes.
2) (acronym) (or more backronym) YIFF or Y. I. F. F. - "Young Incredibly Fuckable Furry"
3) (proper name) YIFF sound server. Program for Linux to help handle audio.
4) (noun) More or less (usually more) sexual act. Commonly used by furries, usually denotes standard intercourse but may denote any kind of sexual behaviour between any type and number of parties (imaginary anthro creatures, humans, animals, even inanimate objects like plushies). Antonym: Yerf.
5) (verb) Like in meaning 4, "to yiff".
6) Furry (or otherwise) artwork depicting yiff as in meaning 4.
7) Expression of joy (by furries)
8) Non-invasive question about sexual feelings/will to mate.

Note in all sexual meanings this has a very specific emotional background: It's neither considered vulgar (like "fuck" or technical (like "have intercourse") or emotion-less (like "make love"). It's playful, considering the act like something fun, nice, pleasant and not very important - without any special obligations attached to it. "Let's yiff" has about the same emotional background as "Let's play".

See also yiffy
1) I heard foxes making the "yiff" noises.
2) He's a Y.I.F.F.
3) I've upgraded the yiff server.
4) Are you into yiff, or just cuddles?
5) Let's yiff.
6) It's hard to find good yiff on VCL nowadays.
7) I did it! Yiff-yiff-yiff-yiff-yiff!
8) "Yiff?" "No, thanks, I don't quite feel like this right now."
by SharpFang August 20, 2003
 
2.
The Furry Fandom's word for Sexual Intercourse, where two furs have a sexual affair. The word originates from the noise a fox creates when it mates. The phrase is used for sexual acts universally involving furries, not just specific acts.
"The lion wanted to yiff the wolf, which was blatantly obvious due to the growing package in his pants, his sheath revealing an ever-shy member."

"The wolf explained to his furry friends about how yiffy he felt by simply standing near his lion friend."

"The canine yiffed the feline in a pleasurable matter, resulting in loud purrs and murrs of great enjoyment."
by Caleb Griffith September 20, 2006
 
3.
1. A happy affirmation in an imaginary fox language, invented by the writer of the Fuzzball MUCK server. The definition was confused with the definition for another word in the same imaginary language, 'yipp', and an administrator of another MUCK heavily promoted 'yiff' word in its first known sexual connotation in 1995 or later. It was only after this point that supposed origins concerning mating sounds and acronyms were invented.

2. Meant to imply feelings of sexual arousal or intent to arouse; also refers to a sexual act. Considered crass language in polite circles.
1. "Yiff!" (Happy affirmation in imaginary foxish language.)
2. "Yiff!" (I am feeling aroused.)
by nobody in particular July 22, 2004
 
4.
(Onomotopia.) A bark made by a fox when aroused. Usually referrs to the sound made by the male of the species.
The yiffs of the foxes in the field signaled a new fox family being born.
by Shanya Almafeta May 09, 2003
 
5.
The Furry Fandom's word for Sexual Intercourse, where two furs have a sexual affair. The word originates from the noise a fox creates when it mates. The phrase is used for sexual acts universally involving furries, not just specific acts.
"The lion wanted to yiff the wolf, which was blatantly obvious due to the growing package in his pants, his sheath revealing an ever-shy member."

"The wolf explained to his furry friends about how yiffy he felt by simply standing near his lion friend."

"The canine yiffed the feline in a pleasurable matter, resulting in loud purrs and murrs of great enjoyment."
by Caleb Griffith September 20, 2006
 
6.
1.Young, Incredibly, Fuckable, Furries
2. A form of internet porn not unlike hentai that involves anthros (bipedal, walkin talkin animals, with human actions, facial expressions, and traits)that is usualy hand drawn
3. The noise a fox makes when mating
4. A very thouroughly slandered fandom throughout the internet by biased self religous try hards
1. Dude, i fapped so hard to that YIFF you sent me.
2. Dude, i couldn't sleep last night, my foxes wouldn't stop yiffing.
by Drake527 November 09, 2011
 
7.
a form of pornography of the furry nature
in which one or more anthropomorphic
animal character(s) is doing something
sexually to another anthropomorphic animal
furry. there are sites such as furaffinity.net
and yiffstar.com where there is a large
selection of yiff(y) stories and pics done
profesionally and amature arts.
(furry 1)wanna yiff?

(furry 2) depends on if you are good at rp

(furry 1) im good if you want me to be *winks*

(furry 2) then lets fuck
by blood wolf May 28, 2009