erstwhile vs once

erstwhile

adj

  • Former, previous. 

  • Respected, honourable. 

once

adv

  • Multiplied by one: indicating that a number is multiplied by one. 

  • One and only one time. 

  • Formerly; during some period in the past. 

conj

  • As soon as; when; after. 

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