backpedal vs invaginate

backpedal

verb
  • To step backwards. 

  • To pedal backwards on a bicycle. 

  • To distance oneself from an earlier claim or statement; back off from an idea. 

invaginate

verb
  • To turn or fold inwardly. 

  • To fold inward to create a hollow space where none had existed, as with a gastrula forming from a blastula. 

  • To fold up or enclose into a sheath-like or pouch-like structure, either naturally or as part of a surgical procedure. 

adj
  • Having one portion of a hollow organ drawn back within another portion. 

  • sheathed 

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