Wiktionary
piss oneself
vb. 1 (context UK slang vulgar literally English) To wet oneself. 2 (context UK slang vulgar figuratively English) To be very scared (to the extent that one might lose control of one's bladder). 3 (context UK slang vulgar figuratively English) To laugh uncontrollably (to the extent that one might lose control of one's bladder).