What is the meaning of Flat_store?

A crooked gambling establishment, such as a casino running rigged or fixed (dishonest) games of chance.

A storage facility in which the material stored is simply piled on a floor (as opposed to a bin or silo).

“flat store”, as which the primary “flat joint” is also known, listed on page 95 of Newspeak: A Dictionary of Jargon by Jonathon Green (1984; Routledge; →ISBN (10), →ISBN (13))

Source: wiktionary.org