knob-end vs mushroom tip

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 

mushroom tip

noun

  • The glans penis. 

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