Yiffing is a combination of words "sniff" and "yell" made up by MatPat's wifeSteph in their livesteam for a game Dayshift at Freddy's, as she didn't know what it actually meant.
MatPat: "So, yiffing is, for those who don't know-"
Steph: "Wait, I should guess! I think that a yiff is probably a yell and a sniff at the same time."