The phrase "be careless with" refers to the act of being negligent or not paying enough attention to something important. Synonyms for this term could include "be reckless with," "be thoughtless with," "be heedless with," "be inattentive to," or "show disregard for." Each of these phrases depicts a lack of care or concern for the task or object at hand, indicating a level of irresponsibility or carelessness. It's important to be mindful of our actions and their consequences, and using these synonyms allows us to describe situations where this mindfulness was lacking. These phrases can be useful in communicating to others the importance of being attentive and responsible.