When someone is "on the defensive," it typically means that they are in a state of being defensive, guarded, or cautious. There are a variety of ways to express this sentiment using different words or phrases. For example, you could say that someone is being "defensive," "guarded," "wary," "apprehensive," "tense," "suspicious," or "cautious." Alternatively, you might describe them as being in a "defensive stance," "guarded posture," or "watchful state." No matter which synonym you choose, the underlying meaning remains the same: the person is feeling protective and defensive about themselves or their actions.