The phrase "am on guard" is usually associated with a state of alertness or vigilance, and can be used to indicate that someone is prepared for potential danger or threats. Antonyms for this phrase could include "relaxed", "unconcerned", "unwatchful", "unprepared", or "complacent". These words suggest a lack of readiness or awareness, and may indicate a situation where one is not taking precautions or preparing for possible risks. It is important to be aware of these antonyms, as they can help convey different shades of meaning and can be useful in a wide range of contexts.