as thin as a lath

You are as thin as a lath and as brown as a nut.

As thin as a lath, if not thinner;

He's as thin as a lath.

That's why he's so fat,' added Josie, with a withering glance at her cousin, who was as thin as a lath. '

In A Study in Scarlet, having just returned from Afghanistan, John Watson is described "as thin as a lath and as brown as a nut."