The phrase "warm a chair" implies someone who is not actively engaged in any task or job. Antonyms for this phrase could include "stand up," "move about," "be productive," "work hard," "collaborate," or "concentrate." These would all be actions that involved more than just sitting in a chair and could connote a sense of purpose, motivation, and productivity. Alternatively, antonyms could focus on the emotional quality of "warm," such as "stay distant," "remain detached," "keep cool," or "lack enthusiasm." These antonyms suggest that someone who is just "warming a chair" is not fully invested or interested in what they're doing.