It is used to describe when you pass something, but with the bare minimum. If you get 50% on a test when you need 50%, then you have finished the test magna cum nada.
2. Man, this year has been so bad. I've really messed things up with my girlfriend, I've been magna cum nada.
3. So I was taking out the trash yesterday for the first time in like a month, and my Mom was saying "You really do stuff magna cum nada".