Prim and proper are terms that describe a person who is excessively formal, correct or stiffly neat. Opposite words of prim and proper are informal, laid back, casual, relaxed, unceremonious, and unorthodox. A person who is informal is easy-going and casual in manner, while someone who is laid-back is tranquil and imperturbable. Similarly, the opposite of proper is inappropriate or indecorous, which refers to behavior or action that is not considered socially acceptable. Unconventional, eccentric or unconventional can also be antonyms for prim proper, meaning not customary, traditional, or conformist. These antonyms add flavor to our vocabulary, enabling more precise expressions and offering us a wider range of language options.