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1.
Usually refers to cyber sex between two or more members of the furry fandom. Originally refers to the sound that foxes make when mating. Members of the furry fandom may use the term to refer to actual sexual contact, usually in a lighthearted manner.

It is also a term used at furcons to inquire about sexual intentions between fursuiters.
The anthro fox smiled at the mass of horny furres, knowing that a good yiffing session would soon follow.
by Suirel Whitedragon January 27, 2006
 
2.
while dressed up in animal costumes, participants grope, fondle, and hump on each other for sexual gratification. no actual sex involved. Usually done in a group setting.
Peter "rabbit" and Jane "cat" fondled each other through their costumes while yiffing at a furry friends party.
by Icerook January 21, 2004
 
3.
When grown ass men, who live with their mothers and own show cats, dress up in animal costumes and dry hump each other.
jon: "Hey Jimmy do you want to go yiff." Jimmy: No, I don't have time for yiffing. I have to go help mother clean the cat cages."
by H8ing you February 07, 2009