Green’s Dictionary of Slang

ring dropping n.

1. [late 18C–19C] a swindle whereby some valuable object is dropped in the road, where it is found by a potential victim. This leads to an encounter, after which the victim is either lured into a fixed game or, in the case of the (fake) valuable, persuaded by the trickster to buy it, claiming that while they should share the profits, he, the con-man, will sell his share and let the victim have the whole benefit; also attrib.

2. [late 19C] a term of scorn, ‘equivalent to “tell your grandmother to suck eggs”’ (Ware); so common had sense 1 become that anyone could spot a ring-dropper.