afoot vs on foot

afoot

adv
  • On foot. (means of locomotion, walking) 

  • On foot. (support of the body, standing) 

  • In motion; in action; astir; stirring; in progress. 

adj
  • That is on foot, in motion, in action, in progress. 

on foot

prep
  • In progress or in preparation; active, in operation. 

  • On one's feet; walking, jogging or running but not in a vehicle or on the back of an animal. 

How often have the words afoot and on foot occurred in a corpus of books? (source: Google Ngram Viewer )