Yiff just simply means to have sex in Furry language. It can also be a label for Furry sexual content in general. Back in the day, the word was coined by fox Furries who heard sounds of foxes mating. Their sound was "Yiff, Yiff, Yiff!" Which reminds me, in the movie Forest Gump, there is this part where Forest as a child is sitting outside of the house and his father is inside with his mother having sex. He makes a loud sound which sounds like "Yiff." After his father comes out, he stands next to Forest wiping his forhead with a cloth. "Boy I tell ya, your mother sure does care about your schoolin'." he says. Then Forest immitates the sound he heard.