"Bet bottom dollar on" is an idiomatic expression often used to indicate certainty or confidence in a particular outcome. While there are no direct antonyms for this phrase, there are several words or phrases that could be used to convey the opposite sentiment. These might include idioms such as "shoot in the dark" or "take a stab at," which suggest uncertainty or risk. Other potential antonyms could include words like "doubtful," "unreliable," or "unsure." Ultimately, the choice of antonym will depend on the specific context and tone of the conversation, but there are many different options to choose from.