dicky (n.)

also dickie, dickey, a diminutive form of dick, used in a variety of senses whose origin, application, and connection are more or less obscure. These include: "detached shirt front worn in place of a shirt" (1811); "a leather apron" (1874); "a donkey" (1793); "a small bird," (1851, short for dicky-bird, a nursery-word attested from 1781); "seat in a carriage on which the driver sits" (1801). For at least the garment senses Century Dictionary suggests Dutch dek "a cover, a horse-cloth."