Wiktionary
flat store
n. (context gambling slang English) A crooked gambling establishment, such as a casino running rigged or fixed (dishonest) games of chance.'''“flat store”''', as which the primary '''“flat joint”''' is also known, listed of '''Newspeak: A Dictionary of Jargon''' by Jonathon Green (1984; ; ISBN 0710096852 (10), ISBN 978-0710096852 (13))