sheep-headed vs short bus

sheep-headed

adj
  • silly; simple-minded; stupid 

short bus

adj
  • Stupid; dumb; slow. 

noun
  • A shorter school bus, typically equipped with a wheelchair lift, used for transporting children who are physically disabled or who are being educated in special programs, often for learning disabilities. 

How often have the words sheep-headed and short bus occurred in a corpus of books? (source: Google Ngram Viewer )