Green’s Dictionary of Slang

a triangle to which a criminal is tied to receive a judicial whipping.

1812		Vaux Vocab. of the Flash Lang. in McLachlan (1964) 260: puzzling-sticks the triangles to which culprits are tied up, for the purpose of undergoing flagellation.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue [as cit. 1812].
1836		A. Thornton Don Juan in London II 404: Lagged means transported; [...] the puzzling-sticks, the cart’s tail, or whipping post.