Urban Dictionary: yiffing
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."
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."
Urban Dictionary: furries
"A subgroup of science fiction and fantasy fans who focus on cartoon animals, anthropomorphic animals, or human-animal hybrids."