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1. 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] 2. Yes indeed, certainly, as in Are you coming tonight?? Sure thing! This use of the idiom as an interjection dates from the late 1800s.
American Heritage Idioms
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Phrase(s): Sure thing.
Inf. I certainly will. • Sue: Will you be at the reception? Bob: Sure thing. • Bill: You remember my cousin, Tom, don’t you? Bob: Sure thing. Hi, Tom.
McGrawhill's American Idioms And Phrasal Verbs