"Under form and stiff" is the phrase typically used to describe something that is overly formal or rigid. However, there are several synonyms that can be used in place of this phrase. One such synonym is "uptight," which implies a tense and constrained demeanor. Another option is "starchy," which suggests a lack of flexibility or spontaneity. "Rigid," "inflexible," and "unyielding" are also suitable synonyms, all conveying an image of something that is firmly set in its ways. Ultimately, the choice of synonym will depend on the context in which it is used, but each of these options can effectively convey the idea of something that is too formal or rigid.