piss one's pants vs relieve oneself

piss one's pants

verb
  • To wet oneself; to urinate in one's own clothes while they are being worn. 

  • To be extremely frightened. 

  • To laugh uncontrollably. 

relieve oneself

verb
  • To urinate or defecate. 

How often have the words piss one's pants and relieve oneself occurred in a corpus of books? (source: Google Ngram Viewer )