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
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."