piss oneself vs shit oneself

piss oneself

verb
  • To be very scared (to the extent that one might lose control of one's bladder). 

  • To wet oneself. 

  • To laugh uncontrollably (to the extent that one might lose control of one's bladder). 

shit oneself

verb
  • To be very frightened or astonished (to the extent that one might lose control of one's bowels). 

  • To soil oneself. 

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