The phrase "on one's last leg" means to be in poor physical condition, near the end of one's life or resources, and close to failure or collapse. Some synonyms for this phrase include "at death's door," "in one's final moments," "on the verge of expiration," "knocking on heaven's door," or simply "about to give out." Other phrases that are similar in meaning are "running on fumes," "hanging by a thread," "on shaky ground," and "down to the wire." Each of these phrases captures the sense of impending loss or demise and suggests a certain fragility or vulnerability.