Fig.: to turn against someone who's helping you; to show ingratitude

Michelle wouldn't comment on her employer's alleged misconduct because - as she put it - she didn't want to speak out of school. He was giving her a job, after all

Michelle non ha voluto fare commenti sul presunto comportamento scorretto del suo capo perché - a sua detta - non voleva sputare nel piatto in cui mangiava: dopo tutto, era il suo datore di lavoro

In this store we treat each of our customers with the greatest respect. That's our motto: never bite the hand that feeds you

In questo negozio trattiamo tutti i nostri clienti indistintamente con il massimo rispetto, perché il nostro motto è: non sputare nel piatto in cui mangi

The metaphor of the dog biting its master was already used about 600 B.C. by the Greek poet Sappho. Today the meaning of this expression refers mainly to people who turn against anyone who helps them