Antonyms for the phrase "spare no effort" include being lax, careless, indifferent, negligent, and apathetic. These words indicate a lack of effort or concern towards a given task or goal, suggesting that the individual may not be fully committed to achieving the desired outcome. Furthermore, these antonyms imply a lack of dedication, enthusiasm, and determination that is required for success. Choosing to "spare no effort" is a mindset that requires a focused and concentrated effort to overcome any obstacles that may arise. Conversely, the antonyms suggest lethargy, complacency, and a lack of commitment that can hinder progress and prevent success.