The proverbial phrase "copperfasten" means, to make a situation or decision very secure or certain. However, there are several other synonyms for the expression "copperfasten," which denote the same meaning. To illustrate, "reinforce" indicates strengthening something that is already in place, while "bolster" refers to supporting or enhancing something vulnerable. "Secure" and "guarantee" acknowledge additional meanings that can convey the same sense of certainty and safety. Other synonyms that can be used instead of "copperfasten" include "strengthen," "certify," "implement," and "establish." Ultimately, the right synonym would depend on the context in which the situation is being described.