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What was a flash house, please?



A brothel, for the wearers of cheap, flashy jewelry who frequent them.



For all I know brothels may have been called flash houses, but that meaning doesn't seem to fit the context:

the complete text

I think it's unlikely that brothels would employ junior clerks. And Poe distinguishes between 'junior clerks of flash houses' and 'upper clerks of staunch firms'. I think 'house' here just means business or company -- as it often did in the 19th century -- and a flash house would mean a probably ephemeral company where the emphasis was on style rather than substance (perhaps we would say flashy these days rather than flash).

Bingley