Source : Webster's Revised Unabridged Dictionary (1913)

Certainly \Cer"tain*ly\, adv.
   Without doubt or question; unquestionably.

Source : WordNet®

certainly
     adv : definitely or positively (`sure' is sometimes used
           informally for `surely'); "the results are surely
           encouraging"; "she certainly is a hard worker"; "it's
           going to be a good day for sure"; "they are coming, for
           certain"; "they thought he had been killed sure
           enough"; "he'll win sure as shooting"; "they sure smell
           good"; "sure he'll come" [syn: {surely}, {sure}, {for
           sure}, {for certain}, {sure enough}, {sure as shooting}]