E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
in George III.s time, meant a flannel petticoat. It was afterwards applied to what were called false shirtsi.e. a shirt front worn over a dirty shirt, or in lieu of a shirt. These half-shirts were first called Tommies.
A hundred instances I soon could pick ye
Without a cap we view the fair,
The bosom heaving alto bare,
The hips ashamed, forsooth, to wear a dicky.
Peter Pindar: Lord Aucklands Triumph.
And sister Peg, and sister Joan,
With scarce a flannel dicky on .
Middlesex Election, letter iv.
(Hair, whalebone, or metal vestments, called dress-improvers, are hung on womens backs, as a dicky is hung on a coach behind.)