When you want to express certainty about something, using the phrase "more sure-thing" can help you convey your confidence. However, there are several other synonyms that you can use to express the same idea. These include terms such as "certain," "definite," "indisputable," "absolute," "positive," and "undoubted." Each of these words can be used in a slightly different context to convey various nuances of certainty. For example, "certain" might be used when you have complete knowledge about something, whereas "positive" might be used when you have a strong belief in something based on evidence or experience. Ultimately, the synonym you choose will depend on the specific situation and the level of confidence that you wish to convey.