The word "un-systematic" refers to something that lacks a proper structure or organization. The antonym of "un-systematic" would be "systematic," which refers to something that has a logical and organized structure. Other antonyms for "un-systematic" could include "methodical," "coherent," "orderly," "planned," "systematized," and "well-organized." These words suggest that there is a clear plan or method to the way something is arranged or carried out, which is the opposite of the disorganized and chaotic nature of something that is "un-systematic." By using these antonyms, one can describe things that are structured and organized in a positive light.