Antonyms for the phrase "slacking off" include terms like diligent, hardworking, industrious, productive, and proactive. When someone is diligent, they are focused, committed, and dedicated to accomplishing their tasks promptly and with precision. A hardworking individual is someone who puts in effort and works actively towards achieving their goals. Industrious individuals are characterized by a devotion to their work, staying busy, and being productive. An individual is productive when they efficiently and effectively complete tasks while generating the desired results. Finally, proactive individuals are always embarking on tasks before they are expected to, working ahead of schedule, and preparing themselves for success. All of these adjectives represent the opposite of "slacking off," emphasizing hard work, motivation, and dedication.