1) A microwave that has been in use for years but never cleaned. Splattered foods from all walks of life are caked on the walls so thick that you could make an entire meal out of it, if you were brave enough to scrape it all off. Regardless of the food you put in there, it comes out smelling like every dish that has been put in before it.
2) What you get when you mix bean burritos, pizza, tuna fish, popcorn, nachos, fish sticks, hot dogs, corn dogs, hamburgers, funnel cakes, macaroni and cheese, beans, chili, Taco Bell, and sloppy joes, put them in the microwave until they explode, and then cook your regular food in it.
3) When something is utterly disgusting.
"Look at my truck. It's got mud and bird shit all over it!" -"Yeah, it looks like Jay's Microwave."
"I had a bad case of explosive diarrhea and when I got done, it looked like Jay's Microwave."
"I picked up a girl at the bar and when I stuck my hand down her pants I got all kinds of Jay's Microwave on it."