The antonyms for "laying groundwork" would connote the absence of preparation, diligence, and forethought. If someone is not laying groundwork, they may be rushing into something recklessly, acting impulsively or haphazardly without considering the consequences. The antonyms may include phrases like "jumping the gun," "rushing headlong," or "winging it." These actions could result in a lack of direction or purpose, leading to chaos, confusion, and even failure. Though it may seem appealing at times to rush into things, taking the time to lay the groundwork and plan accordingly will often be more fruitful in the long term.