Here's why:
* Physical comparison: An onion skin is a very thin layer of the onion, easily torn or broken.
* Metaphorical usage: It's used to describe things like paper that is extremely thin, or even a delicate situation or relationship that could easily be damaged.
Examples:
* "The old map was printed on onion skin paper, so it ripped easily."
* "The peace treaty was a delicate agreement, hanging by an onion skin thread."
Other meanings:
While the "thin and fragile" meaning is the most common, "onion skin" can also refer to:
* Onion-skin paper: A type of very thin paper that was once commonly used for making carbon copies.
* Onion skin disease: A fungal disease that affects onion plants.
To understand the meaning of "onion skin" in a specific context, pay attention to the surrounding words and the overall tone of the sentence.