Definition from Wiktionary, the free dictionary
- Present participle of yiff.
- "never do I ever equate yiffing between furries, whether they be between anthro to anthro, anthro to non-anthro or non-anthro to non-anthro species, where both partners are sentient, to be compared to bestiaitly or zoophilia." — 
- "Some furs are only able to get here at certain times of the week, and they may read your question about yiffing before seeing this message." —