Antonyms for "be conversant in" would be "be unfamiliar with" or "be ignorant of". If someone is not conversant in a particular subject or language, it means that they lack knowledge and understanding of it. Additionally, antonyms could include "be unskilled in" or "be inexperienced in", as being conversant in something implies a certain level of proficiency and expertise. Therefore, if someone is not conversant in a particular area, it suggests that they are not adept at it and lack the necessary skills and experience to tackle it effectively.