"Pre-arranges" refers to the act of setting up plans or arrangements in advance. Some possible antonyms for this word could include terms such as "spontaneous," "impromptu," or "unplanned," which all suggest a lack of prior preparation or organization. Other potential antonyms might include words such as "disorganized," "chaotic," or "uncoordinated," which imply a lack of structure or clarity in terms of planning or decision-making. Ultimately, the choice of antonym will depend on the specific context in which it is being used, as well as the intended meaning or tone of the overall message.