Ambidextrousness is the ability to use both hands equally well. The antonyms for the word would be left-handedness or right-handedness, as these terms refer to a preference for using one hand over the other. Additionally, clumsiness could be considered an antonym, as it implies a lack of dexterity and control. Another antonym could be single-sidedness, which refers to a dependence on only one hand. In general, antonyms for a word refer to concepts that are opposite or incompatible with the original term, indicating a lack of common ground or similarity.