平胸在英语中可以翻译为 "flat chest" 或者 "small breasts". 这个词的来源可以追溯到中文，"平" 在英文中可以翻译为 "flat"，"胸" 在英文中可以翻译为 "chest" 或者 "breasts".
下面是一些例句：
She has a flat chest. (她有一个平胸。)
He noticed that she had small breasts. (他注意到她的胸部很小。)
Some women are self-conscious about their flat chests. (有些女性会因为自己的平胸而感到不自在。)
She wished she had bigger breasts, but she was content with her small chest. (她希望自己的胸部能更大一些，但她对自己的平胸感到满意。)