Sympathetic fiber refers to a nerve fiber that transmits signals from the sympathetic nervous system. The word sympathetic is often associated with empathy, care, and concern. However, if we talk about antonyms for sympathetic fiber, we can use words like unsympathetic, apathetic, cold, heartless, indifferent, and unfeeling. These terms denote a lack of concern, empathy, or warmth. An unsympathetic fiber can lead to lack of emotional responsiveness, difficulty in handling stress, and lowered empathy. Whatever the context of the word sympathetic fiber, its antonyms carry a negative connotation that portray a lack of compassion or feeling.