knap vs nubble

knap

noun
  • A protuberance; a swelling; a knob. 

  • A small hill 

  • A sharp blow or slap. 

  • The crest of a hill 

verb
  • To rap or strike sharply. 

  • To shape a brittle material having conchoidal fracture, usually a mineral (flint, obsidian, chert etc.), by breaking away flakes, often forming a sharp edge or point. 

  • To make a sound of snapping. 

nubble

noun
  • A small knob or lump. 

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