Etymology 2
Likely a portmanteau of squirt and shit.
Noun
the squits
- (plural only, British, colloquial) diarrhoea.
2006, Sarah Waters, The Night Watch, Virago Press, page 290:‘Caroline Graham said you couldn’t make it back upstairs. She laid it on as thick as anything for Gibson―said you’d been taken by surprise by something. Now word’s gone round you’ve got the squits.’