safety pin vs treenail

safety pin

noun
  • A pin in the form of a clasp, which has a guard to cover the sharp point; used to join two pieces of fabric, etc., together temporarily. 

  • The pin of a hand grenade, which prevents accidental detonation. 

treenail

noun
  • A wooden peg or pin used as a fastener. 

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