"Shanks' nag" is an old-fashioned term that describes the act of walking due to the absence of a horse or other means of transportation. Some synonyms for this phrase include "hoofing it," "trekking," "strolling," "rambling," and "sauntering." These are all different ways to describe on-foot travel and can be used interchangeably depending on the context. "Shanks' nag" is often used to emphasize the physical act of walking, making it a vivid and descriptive phrase. However, using different synonyms for this phrase can add color and variety to your writing or conversation, giving you more ways to express your ideas.