Yiffscript is a mIRC script file for generating "/me" style action commands on an IRC channel. It was originally programmed by Dr. Dos, but was soon handed over to SquareMoogle. This file only works on the IRC program mIRC for Windows.
It was intended for use on the CYD IRC channel, but it's overuse (by people other than the creator) led to a few temporary bans, and an unwritten policy of sparing use of the script in that particular channel.
The script overrides the user's "/action" command (i.e. "User: /me goes for a walk" becomes "User goes for a walk",) with randomly chosen "yiffy" phrases based (originally) on real tinysex logs, substituting the names of the user and another member of the channel for those in the base text.