Green’s Dictionary of Slang

1. a citizen of London; thus a tradesman.

1600		T. Heywood Edward IV (1874) I 18: Flat-caps thou call’st us. We scorne not the name.
c.1611		Dekker Match Me in London I i : king: What’s her Husband? lad: A flatcap, pish.
1617		Middleton & Rowley A Fair Quarrel IV iv: Old flatcaps or young heirs.
c.1629		R. Brome City Wit IV i: Oh twas a notable dull Flat-Cap.
1700		‘The Growth of Cuckoldom’ in Playford Pills to Purge Melancholy II 109: If you walk the Town of London, / Where the Flat-caps call Men Cousins.
1705–07		N. Ward Hudibras Redivivus II:3 9: Young Flat-caps, with extended Throats, / Crying their Damsons, Pears, and Nuts.
1719		in D’Urfey Pills to Purge Melancholy IV 109: [as cit. 1700].
1722		‘Whipping-Tom’ Universal Poison, or the Dismal Effects of Tea II 12: Even Women who cry Grey-Pease [...] Flat-Caps, Bunters, and all the Scum of the Nation, cannot go to Break-fast without a Dish of it.
1822		R. Nares Gloss. (1888) I 312: flat-cap. A term of ridicule for a citizen. In Henry the Eighth’s time flat round caps were the highest fashion; but as usual, when their date was out, they became ridiculous. Citizens of London continued to wear them, long after they were generally disused, and were often satirised for it.

2. a Billingsgate fishwife.

1698		N. Ward London Spy II 40: Round the Fire sat a tatter’d Assembly of fat Motherly Flat-Caps, with their Fish-Baskets hanging upon their Heads.
1768		Gentleman’s Bottle-Companion 1: At which the flat-cap form’d a smile.
1859		(ref. to c.1700) G.A. Sala Twice Round the Clock 208: The Market Women’s, or ‘Flat-cap Club,’ was at one time quite a fashionable place of meeting.