Antonyms for the phrase "checking up" could include words such as ignore, neglect, or overlook. These words imply a lack of attention or concern for something or someone. Another antonym for checking up could be passivity, which indicates a sense of indifference or apathy towards the situation. Alternatively, some antonyms could indicate an active opposition to checking up, such as sabotage, disrupt, or defy. Regardless of the specific antonym used, the underlying meaning of these words indicate a lack of interest in carefully monitoring, assessing, or supporting a particular person or task.