The phrase "like putty" refers to something that is easily influenced or molded. Its antonyms, therefore, would describe qualities that are not easily manipulated or changed. Antonyms for "like putty" could include words such as resolute, unyielding, steadfast, unbending, or inflexible. These words suggest a sense of strength and determination, qualities that cannot be bent or shaped to fit someone else's desires. Conversely, people who are "like putty" may be seen as weak, indecisive, or easily swayed. By using antonyms such as these, we can better describe the qualities of someone who is firm in their beliefs and actions, and not easily influenced by others.