Sensible Vocabulary: similar terms and phrases Certainty (as) obvious as day idiom (as) certain as eggs is eggs idiom and that is flat! idiom guarantee someone of something certain decidedly deffo definite surely definitive inviolably ironclad irrefutable irrefutably make no miscalculation (about this) idiom unquestionably unreserved unreservedly watertight would See https://stephenmkeys.ssnblog.com/27722071/rumored-buzz-on-flat