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

I couldn’t care less (but one must keep up appearances, right?)

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.

to yell and sniff near-simultaneously

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.

(Onomotopia.) A bark made by a fox when aroused. Usually referrs to the sound made by the male of the species.

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.