Brewer's: Thin as a Whipping-post
As a lath; as a wafer. (See Similes.)
“I assure you that, for many weeks afterwards, I was as thin as a whipping-post.” —Kingston The Three Admirals, chap. vi.
“ `I wish we had something to eat,' said Tom. `I shall grow as thin as a whipping-post ... I suspect.' ” —