The term "beefed-up" connotes strength, power, and bulk. Therefore, its antonyms represent concepts of weakness, powerlessness, and slimness. The opposite of beefed-up can be skinny, frail, weak, malnourished, or puny. These antonyms can be used to describe individuals who are physically weak, underweight, and lack strength. The word "beefed-up" can also be used to describe objects or situations that are made stronger or more robust. Therefore, the antonyms for it can be feeble, flimsy, and unsubstantial. These antonyms can be used to describe objects that lack stability, durability, or strength.