The antonyms for the phrase "becomes limp" are words that describe rigidity, firmness, and stability. Some antonyms could include words such as stiffen, tighten, strengthen, or harden. These words imply a state of solidity and durability, which is the opposite of becoming limp. Other possible antonyms include words like fortify, solidify, toughen, or coagulate, which all suggest a process of becoming harder and more resistant. These antonyms are useful when describing materials that need to remain firm or sturdy, such as building materials or fabrics. When discussing physical or mental states, such as motivation or resolve, antonyms like energize, invigorate, or steel are more appropriate.