When you're drunk and can't think straight.
Dude, I just downed 11 beers, got a case of beer brain.
A consequence of continual beer chat whereby the brain's beer neurons fire relentlessly at the salivary glands, producing persistent, excessive salivation and a recurrent desire to hit up the local watering hole.
Dude: ...yeah, and I couldn't believe they quintuple-hopped the second batch!
Bro: Totally. It's like--oh, god...I can't stop drooling...it's filling up my mouth...are we near a Dogfish Head?!....
Dude: Man, you've got some serious beer brain.