1. Mentally or physically infirm with age
   "His once-sharp mind had gone gaga";
   - doddering, doddery, senile
    
2. Intensely enthusiastic about or preoccupied with
   "gaga over the rock group's new album";
   - crazy [informal], wild [informal], dotty [Brit, informal]

See also: enthusiastic, keen, old

Encyclopedia: Gaga, Central African Republic