The phrase "slack off" means to reduce effort or activity or to become lazy. The antonyms for "slack off" would be to increase effort, work harder or to become more productive or active. Some of the antonyms for "slack off" would be to strive, work harder, concentrate, focus, redouble, intensify, hustle, accelerate, expedite, push oneself, get busy, buckle down and be diligent. These words can be used as a reminder for individuals to focus on their goals and to maintain their productivity without being distracted. By avoiding the temptation to slack off, they can achieve success and accomplish their goals.