The phrase "most honest-to-goodness" refers to something that is authentic and genuine. The antonyms for this word can be anything that is fake, fraudulent, or counterfeit. These antonyms can include words like fake, phony, deceitful, disingenuous, bogus, insincere, hypocritical, and untrue. These words represent the opposite of what most honest-to-goodness stands for. They imply that something is not authentic or true, and is instead a manipulation or fabrication. Therefore, when you want to describe something that is not sincere, you can use any of these antonyms and they will effectively convey the opposite meaning of most honest-to-goodness.