Mum No Cook - It refers to your mum is not making dinner tonight. please make/buy your own dinner.
Hey, tonight MNC. settle your dinner before you come home.
Mythical National Champion - It refers to the fact that College Football needs a playoff system and until that happens, there will be no real "Champion", just a "Mythical National Champion".
If Florida beats Ohio State, then they will be the MNC because all other 11-1 (or 12-1) teams should of had a chance.