bethink vs think back

bethink

verb
  • To think of (something or somebody) or that (followed by clause); to remind oneself, to consider, to reflect upon. 

  • To meditate, ponder; to consider. 

  • To determine, resolve. 

think back

verb
  • To think about a time or experience; to recall. 

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