Yiff is an artificial word, created by and for the furry fandom.
The term may refer to pornographic content, involving or depicting furries, as well as the sexual intercourse itself, done by furries.
While it may seem weird and out of the order, the term itself is usually treated with humor and is not taken seriously.
“Yiff” was initially meant to be an onomatopoetic, Foxish word, used for greeting other furries, as well as expressing excitement and affirmation to something, as a general positive response.
The Foxish language comes from the biggest online MUD, called FurryMUCK, where it was primarily used by fox roleplayers.
The erotic meanings of “yiff” came from the common mistake of many users, who mistakenly mixed up the term with “yipp” a word, which possesses sexual connotations.
The expression “yiff” was presented to a wider audience in 2003, when the CSI episode, titled “Fur and Loathing” was aired.
Here, the word “yiffing” is mentioned several times, although the episode is criticized by many furries, because it implies, that all furries are fetishists.
The term was first defined on Urban Dictionary in 2003, with several other definitions explaining it since then.
The ever growing furry community is ensuring, that the expression won’t disappear from the English language, rather it gets more and more embedded into the vocabulary.