"Up on one guard" is a phrase often used in sports and martial arts to describe a defensive stance, where a person is poised to react quickly to an attack. There are several synonyms that can be used to describe this stance, including "on the defensive," "cautious," "vigilant," "alert," "watchful," "prepared," "ready," "armed," "on guard," and "alert to danger." It is important to maintain a defensive stance in order to protect oneself from any unexpected attacks, and using synonyms can help to convey this message effectively. Whether on the field or in battle, being "up on one guard" means being fully prepared to defend oneself.