From Longman Dictionary of Contemporary Englishdrop dead dateˌdrop dead ˈdate noun [countable usually singular] American English informal a date by which you must have completed something, because after this date it is no longer worth doingFrom Longman Business Dictionarydrop-dead datedrop-ˈdead date American English informal a date by which it is very important that you have done or completed something, because after this date it is no longer worth doing or completingThe original drop-dead date is June 23, but if all parties agree, there could be an extension. → date
