Hmm... this article[1] cites an interview with Mike Judge where he says it mean "Test Program Set" when he was an engineer.
Anyway, I've seen more than a few people jump to the conclusion that it stands for something to do with the "Toyota Production System" which is unfortunate.
That's too bad. It's quite funny thinking Toyota Production System is where TPS reports come from because it's so believable. Even though Toyota switched to Agile-ish methodologies, they still produce 20x the documentation and paperwork that the Waterfall method produces!
Anyway, I've seen more than a few people jump to the conclusion that it stands for something to do with the "Toyota Production System" which is unfortunate.
[1]: http://mentalfloss.com/article/57338/what-tps-report