Green’s Dictionary of Slang

flash adv.

[flash adj. (1)]

1. ostentatiously.

[UK]‘A Rum-Un to Look At’ in Libertine’s Songster in Spedding & Watt (eds) I 136: Her valk and her beat, / Is up Coventry-street / Togg’d flash from her head to her toe.
[UK]Swell’s Night Guide 66: Pays the gorger twopence, and tips it flash, (spins it over to him) ‘Vell, culley I’m jest stalling over to the lush crib to a pal, for a drain of hevy’.
[UK]Empire (Sydney) 27 July 5/5: ‘A Botany Bay Bagman’ / Says they, with lots of cash / We saw on Epsom Course / A coming of it flash.
[Aus]Truth (Sydney) 16 Dec. 8/3: Togged up orful flash and neat.
[Aus]Sport (Adelaide) 21 June 9/1: They Say [...] That Harry M. [...] is doing it flash with the Thebarton girl in a short dress.

2. boastfully, arrogantly, in a showing-off manner.

[UK]F. Norman Bang To Rights 93: This geezer comes into my peter and starts talking flash.
[UK]G.F. Newman You Flash Bastard 71: Few CID cared for suspects running flash, and even fewer stood it when chummy was black.

In phrases

cut (it) flash (v.)

(Aus.) to show off.

[Aus]Sport (Adelaide) 24 Aug. 14/1: They Say [...] That Din, with his new ‘Sac and Bonnet,’ with a nobby up the back, and new ‘daisy roots,’ is cutting flash.
[Aus]Sport (Adelaide) 5 Mar. 5/5: The school teachers didn’t cut it flash with their Johnnies [...] on Saturday.