One whom has not the milk to fill one's bowl of cereal, thusly substituting beer instead.
Dude 1 : "Man, I'm freaking drunk. And hungry! I'm gonna have some cereal!"
Dude 2 : "... But dude... We're outta milk!"
Dude 1 : "Ah shit dude... WELL... there's beer!! Fuck it! I'll just have some beereal!"
Cereal with beer rather than milk. Common darty-pregame breakfast.
I'm ready bro, I started the day with some beereal.