flip-flop vs hark back

flip-flop

noun
  • An instance of flip-flopping, of repeatedly changing one's stated opinion about a matter. 

  • A return trip. 

  • A sandal consisting of a rubber sole fastened to the foot by a rubber thong fitting between the toes and around the sides of the foot. 

  • A bistable; an electronic switching circuit that has either two stable states (switching between them in response to a trigger) or a stable and an unstable state (switching from one to the other and back again in response to a trigger), and which is thereby capable of serving as one bit of memory. 

  • A change of places; an inversion or swap. 

  • The sound of a regular footfall. 

  • A somersault. 

verb
  • To alternate back and forth between directly opposite opinions, ideas, or decisions. 

hark back

noun
  • An act of alluding, returning, or reverting (to a subject previously mentioned, etc.); also, an act of evoking, or longing or pining for (a past era or event). 

  • An act of hounds retracing a course in order to pick up the lost scent of prey. 

verb
  • To allude, return, or revert (to a subject previously mentioned, etc.); also, to evoke, or long or pine for (a past era or event). 

  • To return to where one has previously been; to retrace one's steps. 

  • To call back (hounds); to recall. 

  • Of hounds: to retrace a course in order to pick up the lost scent of prey. 

How often have the words flip-flop and hark back occurred in a corpus of books? (source: Google Ngram Viewer )