1. yiff - Noun
2. yiff - Verb
3. yiff - Interjection
(onomatopoeic, apocryphal) Representing the bark of a fox (especially while mating).
(of a person, informal) To express happiness, to state that something is sexy.
(slang, informal) Sex, especially between furries (fictional anthropomorphic animal characters, and/or members of the community surrounding their celebration).
"FurryMuck yiffs are like real-life yiffs - there is a time and place for everything." — [1]
(slang, informal) Pornography of or involving furries.
Do you draw yiff?
(transitive, intransitive, slang, informal) To have sex, to mate (said of animals, especially foxes, or people dressed up as animals).
(transitive and intransitive, slang, informal) To propose cybersex to someone.
Source: en.wiktionary.org