His classmates called him 'fattie' when he was a kid, but now he doesn't mind the nickname at all.

fattie 是由形容词 fat（胖的）加上名词后缀 -ie 构成的名词，后缀 -ie 常用来表示昵称或小称，带有亲昵或调侃的语气。fattie 指"小胖子"，是一种非正式但通常不带恶意的称呼。