Молчание — знак согласия. Jill: What did Fred say when you told him we were thinking about leaving the office early?
Jane: He didn't say anything.
Jill: Then he must not mind if we go. Silence gives consent.
полное молчание, молчать как мертвый
(так же: silent as the grave) I knew something was wrong as soon as I entered the classroom; everyone was silent as the dead.
Jessica is as silent as the grave on the subject of her first marriage.