knob-end vs purple helmet

knob-end

noun

  • The glans penis 

  • Used other than figuratively or idiomatically: see knob, end. The end of something that is shaped into a knob. 

  • A stupid or contemptible person 

purple helmet

noun

  • The glans penis. 

How often have the words knob-end and purple helmet occurred in a corpus of books? (source: Google Ngram Viewer )