From Longman Dictionary of Contemporary Englishhook, line, and sinkerline, and sinker hookhook, line, and sinkerBELIEVEif someone believes something hook, line, and sinker, they believe a lie completely → hook
Examples from the Corpus
hook, line, and sinker• The media bought Stuart's story hook, line, and sinker.• The people seem to have swallowed the government's promises hook, line, and sinker.• What an idiot! He believed the whole story hook, line, and sinker! 