There are a lot of origin stories out there that sound really good but aren’t true. You might say such phrases are for the birds.
The phrase “thin as a rail” falls into this category.
This one came on our radar recently thanks to a colleague of Professor Anne Curzan, who happens to study avian evolution and ecology. Basically, he’s a bird expert.
This colleague wanted to know whether “thin as a rail” comes from a type of marsh bird that’s actually called a rail. He noted that rails are very thin and capable of walking through narrow spaces between cattails.
That makes total sense and would be a great origin story, if only it were true.
The “rail” in “thin as a rail” refers to a bar or a piece of wood or metal that’s used for support. It originally referred to part of a fence, but now apply to the rails on stairways, railroads, cars, etc.
The earliest evidence we have of this phrase goes back to 1795, according to the Oxford English Dictionary. It was originally “lean as a rail,” as seen in the example the OED cites: “As dull as a cat and as thin as a rail.”
It’s plausible that the bird theory started here. Someone may have assumed that since the saying includes a cat, this “rail” must be a bird.
The bird theory doesn’t hold up though, since the expression “lean as a rake” has been around since the 1400s. By the 1500s, a “rake” could refer to a very thin person.
Early on, “lean (or thin) as a rake” was the more common expression, until around 1900, when it was surpassed by “thin as a rail.” The latter phrase has remained the more common of the two, though “thin as a rake” is still in use.
