thin as a rail

Extremely skinny or slender. Have you seen Claire lately? She's become as thin as a rail in the last six months! I've always been thin as a rail, even when I try to pack on some muscle.

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]