There are several synonyms for the phrase "under restraint," which means to be restricted or constrained in some way. One common synonym is "restricted," which implies a certain degree of limitation or control over one's actions. Other synonyms include "confined," "constrained," "repressed," "suppressed," and "controlled." Each of these words has slightly different connotations, but they all convey the idea of being limited in some way. Whether due to physical or psychological factors, being under restraint can be a frustrating and challenging experience, but sometimes it's necessary for one's safety or the safety of others.