Green’s Dictionary of Slang

stone n.5

[rhy. sl. stone jug = mug n.1 (2a)]

(UK und.) a certainty, in terms of gullibility, a sucker n.1 (3a)

[UK]Mirror of Life 27 July 14/2: To the slopes of Alexandra Park There came one day a stone. / He came there with his sparklers, And he came there all alone.