In old books, I keep coming accross the saying,
...is so transparent it could pass through the proverbial wedding ring.
What does this mean?
Some makers of some fine, light cloths — or the garments made of them — will boast that because they are so fine, you can roll them up and pass them through a wedding ring. (You are supposed to be able to do this with a Shetland shawl, a superfino Panama hat, a ring pashmina [see where the name is from?] and other items.)
The proverbial in this case means "often talked about in a common idiom, saying or cliché", so it's directly addressing the fact that other people have used the same statement before.
Personally, I dislike this use of proverbial: It's almost like saying "I'm going to write or say something corny and tired now, but that's okay because I'm pointing out that I know it's corny and tired". When I find myself using it, I try not just to cut it, but to cut the whole passage and re-write.