The phrase "on the shelf" refers to something or someone that is not being actively used or pursued. There are several synonyms that can be used in place of this phrase, depending on the context. For products, items or goods that are not being bought or consumed, alternative phrases could include "unsold," "unavailable" or "untouched." For people who are not currently employed or engaged in a particular activity, other options might include "idle," "inactive," or "unemployed." In any case, the synonyms used should be chosen based on the particular situation and what is most appropriate to convey the intended meaning.