Flat

Flat meaning an apartment is a British colloquialism that dates from around 1824. From the late 18th/early 19th century, flat was commonly used to mean a floor or a storey of a house. By association, it came to mean a single-storey dwelling or apartment. The word apartment itself means a separate suite of rooms in a house, ‘apart’ from the rest of the house, and dates from the early 18th century. Flat meaning dull and unenergetic dates from the late 16th century as does the musical term as in sharps and flats. Flat, as in ‘a flat no’ or ‘a flat-out no’ meaning an unequivocal ‘no’ dates from the late 19th century, and this meaning of ‘flat’ as downright, absolute or unqualified, probably derives from the original meaning of ‘flat’, as in ‘a flat landscape’ meaning a landscape without any elevation, which dates from the 13th century. In America, the latter is more likely to appear as a ‘flat-out no’, where out is merely an intensifier, probably deriving from the expression out and out.