(informal)be very knowledgeable about something(非正式)（在某方面）知识渊博的
英语四级真题
Experts observe that textbooks come in layers, something like an onion.
2019年12月四级真题（第三套）阅读 Section C
柯林斯高阶英汉双解学习词典释义
N-VAR
洋葱；洋葱头
An onion is a round vegetable with a brown skin that grows underground. It has many white layers on its inside which have a strong, sharp smell and taste.