Green’s Dictionary of Slang

in an ostentatious, showy manner.

1830		Lytton Paul Clifford I 35: ‘The gemman vot writes,’ said Dummie, putting his finger to his nose, – ‘the gemman vot paid you so flashly!’.
1857		‘Leary Man’ in ‘Ducange Anglicus’ Vulgar Tongue (1857) 42: Your fogle you must flashly tie, / Each word must patter flashery, / And hit cove’s head to smashery, / To be a Leary Man.