Re-activeness refers to the ability to respond quickly to any situation. The antonyms for the word re-activeness include unresponsiveness, apathy, lethargy, sluggishness, and inactivity. Unresponsiveness refers to a lack of reaction or response to a particular situation. Apathy points to a lack of interest, enthusiasm or feeling towards something. Lethargy implies a state of slow, sluggish inactivity or drowsiness. On the other hand, Sluggishness refers to a lack of energy or speed, while inactivity refers to a state of inaction or idleness. So, while a reactive person is sensitive to his surroundings and takes action quickly, an unresponsive person lacks the ability to do so.