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.

It is also a term used at furcons to inquire about sexual intentions between fursuiters.

I couldn’t care less (but one must keep up appearances, right?)

Yiffing is a combination of words "sniff" and "yell" made up by MatPat's wife Steph in their livesteam for a game Dayshift at Freddy's, as she didn't know what it actually meant.

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.

When grown ass men, who live with their mothers and own show cats, dress up in animal costumes and dry hump each other.