"Most maidenly" refers to a quality that is prim and proper, characteristic of a demure and modest young woman. However, the antonyms for this phrase would be words that describe behavior and characteristics that are opposite to these. These could include words like bold, audacious, forward, promiscuous, provocative, outgoing, and flirtatious. These words describe individuals who are not afraid to express themselves, sometimes even boldly defying social conventions. While "most maidenly" is often used to describe women, the antonyms can apply to both genders and can be used to describe individuals who are confident, unafraid, and willing to take risks.