Read it here.
Wiktionary currently has the the most hilarious etymology I have ever seen. "Yiff was part of a range of onomatopoeic words that form a pseudo-language used by the furry role-playing community: yiff, yip, yerf, yaff, yarf, growf and growlf (in order from most positive connotations to most negative connotations). Yiff meant yes or an exuberant hello! Later, yiff was assigned a meaning of a sexual proposition, a meaning that had previously been assigned to yipp (a coarse form of yip). In modern use within the furry community, yiff almost exclusively refers either to sex between anthropomorphic animals, or pornographic depictions thereof."
Take a vacation in Boringdon Hall.
Another great Seanbaby Article: 5 Annoying Things They Don't Tell You About Being A Parent.