thin as a rail
Very slender, as in I do not know why she's dieting; she's thin as a rail already. This simile, which uses rail in the sense of “a narrow bar,” has largely replaced such other versions as thin as a lath or rake, although the latter is still common in Britain. [Second half of 1800s]
Words nearby thin as a rail
thimblewit,
thimbu,
thimerosal,
thimphu,
thin,
thin as a rail,
thin client,
thin edge of the wedge,
thin film,
thin on top,
thin register