From Longman Dictionary of Contemporary Englishdeadly serious/dull/boring etcdeadly serious/dull/boring etcVERYvery serious, dull etc I’m deadly serious, this isn’t a game! → deadly
Examples from the Corpus
deadly serious/dull/boring etc• And at a time which - surely it was obvious - was deadly serious.• His companion chuckled at the jest, but Gravelet, whose stage name was Blondin, was deadly serious.• It was now clear, however, that the position was becoming deadly serious.• Suppose, for example, you regularly attend a weekly meeting which tends to be deadly dull.• The noise level was high in both languages; all faces were deadly serious.• He was deadly serious and I knew it.• The primary indicator is Attempts to be deadly serious invariably result in unintended comedy.• He's a deadly dull little man as far as I can see. 