"Like one who has a ramrod down his back" is a phrase used to describe someone who is extremely straight and rigid in their posture or behavior, often indicating a sense of formality or strictness. However, there are many different synonyms for this phrase that can be used in different contexts or to convey different shades of meaning. Some possible synonyms might include "stiff as a board," "starchy," "formal," "buttoned-up," "uptight," or "unbending." Each of these words has its own connotations and nuances, so choosing the right term will depend on the context and the specific meaning that you are trying to convey.