Green’s Dictionary of Slang

1. to swagger.

1824		J. Hogg Justified Sinner 236: There was no flaring, no flummery, nor bombastical pretensions.
1845		J. Lindridge Sixteen-String Jack 125: Chrissy odsbuds, I’ll on with my duds, / And over the water we’ll flare [...] For still I’ll dance it, prance it, dance it, / Flaring away with Kit!

2. (US) to set alight, to turn on.

1859

Calif. Police Gazette 24 Apr. 1/3: ’Ere, ’ere! I say, flare the glim!

3. to behave excessively, thus in poor taste.

1878

‘’Arry on ’Igh Life’ in Punch 20 July 24: Them Hupper ten Toffs [...] do appear to be flarin’ it fine.

4. to steal by sleight of hand.

1861

(con. 1840s–50s) H. Mayhew London Labour and London Poor I 411/1: Just after that I flared it (whisked the handkerchief out); and that’s the first I did. It brought 1s. 3d.