The term "reckoned on" means to rely or depend on something, but there are several antonyms that can be used to convey the opposite meaning. One such antonym is "unreliable," which means not trustworthy or not able to be depended on. Another antonym is "unexpected," which suggests that something is surprising or unforeseen, making it impossible to rely on. "Undependable" is another antonym, meaning that something or someone cannot be counted on. Lastly, "uncertain" is an antonym for "reckoned on" which implies doubt or lack of confidence in a particular outcome or event. Using these antonyms can help convey a sense of unpredictability or lack of dependability.