1. something that is or is supposed to be a certain success, as a bet or a business venture: He thinks that real estate is a sure thing.
2. something assured; certainty: It's a sure thing that he'll refuse to cooperate.
3. surely; for sure; O.K.; roger (often used as an interjection).
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.