The phrase "twisted arm" refers to an injury or discomfort in the arm caused by twisting it too hard or in a wrong position. The antonyms for this phrase can be "uninjured arm," "straight arm," "flexible arm," or "un-twisted arm." An uninjured arm is one that is free from any pain or injury, unlike a twisted arm. A straight arm is one that is positioned normally without any twisting. A flexible arm is one that can move easily without any pain or discomfort, unlike a twisted arm. Finally, an un-twisted arm is one that has not been subjected to any twisting and is in a normal position.