Colder than a welldigger's...

There are many occupations which steel the person because of unending exposure to tragedy and unhappy situations. This expression is perhaps less used today with digging machines but I wonder what the background of it might be...........?????????


'Harder than a gravediggers heart', or is it 'Colder than a gravediggers heart'

"Colder than a welldigger's a**." But that's about temperature, not emotions.