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"
In a way which is certain; with certainty.
"She certainly knows her way around the city, having lived there for twenty years without ever getting lost."
In plain English: Certainly means something is definitely true or will definitely happen without any doubt.
"I certainly hope you have a wonderful day."
Usage: Use certainly to express strong agreement or to state that something will definitely happen without doubt. Place it before the main verb or after the subject and auxiliary verb in a sentence to emphasize your conviction.
The word certainly comes from Middle English, where it was formed by adding the suffix "-ly" to the adjective certain. It entered modern usage with the straightforward meaning of "in a certain manner," which remains its definition today.