This customer has been loud, rude, and demeaning since they arrived. I'm planning to serve them very old meat from the back.
That's a pretty terrible thing to do, why would you do that?
It's not bad in this situation
Serving old meat to the customer could cause extreme illness in them. Instead of serving the customer the meat you should try to find an alternative meal for them. Serving the customer old meat will result in you most likely getting fired.
F*ck them?
It's normal to get frustrated with rude and obnoxious customers, but this kind of revenge is dangerous. The meat could make them violently sick. Do you really want that on your conscience?