The Collaborative International Dictionary
Flash house
Flash \Flash\, a.
Showy, but counterfeit; cheap, pretentious, and vulgar; as, flash jewelry; flash finery.
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Wearing showy, counterfeit ornaments; vulgarly pretentious; as, flash people; flash men or women; -- applied especially to thieves, gamblers, and prostitutes that dress in a showy way and wear much cheap jewelry.
Flash house, a house frequented by flash people, as thieves and whores; hence, a brothel. ``A gang of footpads, reveling with their favorite beauties at a flash house.''
--Macaulay.
Wiktionary
flash house
n. (context obsolete English) A brothel.