Welcome from me too!
Terry is very well read and has often admitted that he doesn't remember where he got a phrase or a name from (Rincewind being a good example). Chances are he read it somewhere.
Interestingly, I just Googled the phrase and Urban Dictionary says it's a Northern English phrase, while another site says it's Kentish.
“Men never commit evil so fully and joyfully as when they do it for religious convictions.” – Blaise Pascal