slang term, huevos means eggs or testicles, en tu arroz means rice,there is a meal in south America called eggs on top of rice, "huevos en arroz", in this slang meaning "my balls on your rice"
when you don't believe something that somebody is telling you or denying something.
Oye que crees que me paso ayer? me gane la loteria, ay si... mis huevos en tu arroz.
Me prestas tu coche? no, mis huevos en tu arroz que te lo presto. = guess what? I won the lotery yesterday, Yeah right dude!
Can i borrow your car? no you got to be crazy if you think that's happening.