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"
Without fail.
"The sun rises surely every morning without exception."
Surely comes from Middle English as a combination of the word sure and the suffix -ly. It was formed to create an adverb meaning "in a certain or confident manner."