spokenused to show you cannot believe that something is true:
'The chairman's just handed in his resignation.' 'Surely not.'
Such sinners will surely be punished.
4American Englishold-fashionedused to say 'yes' to someone or to express agreement with them
➔ slowly but surely
at slowly (2)WORD CHOICE: WORD CHOICE: surely, definitely, certainly, naturally, be sure toUse surelyto say that you think something must be true, especially when other people do not agree• They should be there by now, surely.Use definitelyto say that something is certain to happen or be true• I will definitely be back (NOT I will surely be back) by ten. Use certainlyto emphasize that something is true• He certainly is (NOT surely is) a great cook.Use naturallyto mean 'as anyone would expect'• They were very late, so naturally I was (NOT surely I was) worried.Use be sure toto say that something is certain to happen• If you work hard you are sure to succeed (NOT you will surely succeed).
Definition from the Longman Dictionary of Contemporary English Advanced Learner's Dictionary.