To quarrel with your bread and butter is the give up your job without sufficient reason, which is foolish.
‘Our new manager’s perfect pig!’ said Janet. ‘Even if I slip out for half an hour or so to do some shopping, he wants to know where I’ve been and why it took so long. If he doesn’t change his tune I’m giving in my notice.’
‘You might not get so good a position next time,’ advised her friend. ‘I shouldn’t quarrel with my bread and butter if I were you.’