|1.||chiefly (US) (sentence substitute) all right! yes indeed! used to express enthusiastic assent|
|2.||something guaranteed to be successful|
a sure thing. A certainty, as in Making the bestseller list has been a sure thing for Stephen King. This usage originally alluded to a bet that one could not lose. [First half of 1800s]
Yes indeed, certainly, as in Are you coming tonight?Sure thing! This use of the idiom as an interjection dates from the late 1800s.