WordWeb Online

1. Showing a lack of intelligence or thought; stupid and silly
   - asinine, fatuous, inane, mindless
    
2. Devoid of significance or point
   "vacuous comments"
   - empty, hollow
    
3. Devoid of matter
   "a vacuous space"
    
4. Void of expression
   "a vacuous stare"
   - blank

See also: daft, dumb, empty, foolish, incommunicative, meaningless, nonmeaningful, stupid, uncommunicative, vacuity, vacuously, vacuousness

Encyclopedia: Vacuous