The character '尼' is derived by changing the end of the word representing female in Sanskrit, not the meaning of Ama (Bikuni (female Buddhist disciple)) representing a female practitioner of austerities.
「尼」の字は、女性の出家修行者をあらわす尼（あま、比丘尼）の意味ではなく、女性をあらわすサンスクリット語の語尾が変化したもの。