A slang term for the use of the Polymorph spell, because it changes the target into a sheep. A caster who uses the Polymorph spell may be referred to as a sheeper.
If sheeping is broken for any reason (such as an AoE skill or another inadvertent attack), the sheeper should know better than to try sheeping that same target, especially if they are currently being attacked and it appears obvious. Then the sheeper should tell the person that is violating this and remind them to stop doing so. After all, there is probably a target that more people are attacking so that they can kill that one faster.
See also Crowd Control.