Some synonyms for the term "sure-fire proposition" can include words like "guaranteed success," "certainty," "no-brainer," "slam dunk," "foolproof," "assured," "solid bet," and "safe bet." All of these words bring the same level of assurance and confidence to a proposition or idea. When you are looking for something that is a sure thing, you want language that instills confidence and certainty, and these synonyms all accomplish that task. Whether you are making a proposal to investors, planning a new business venture, or simply trying to convince a friend to join you on a new adventure, these words can help convey your level of confidence and reassurance.