(NOT ACTIVE) [adjective] flatter, flattest - not active; not interesting or lacking emotionAfter the excitement of the party, life seems very flat now.We thought the leading lady was excellent, but the rest of the performances were rather flat.She answered his questions in a completely flat tone (= her voice expressed no emotion or interest).Business is always a bit flat at this time of year (= not much is being sold).I think the colours in this painting are rather flat (= not varied or bright).(UK and ANZ) I left my car lights on all night and now the battery is flat (US dead) (= has no electrical power left in it).A drink which is flat has stopped being fizzy.If you don't put the top back on that bottle of beer, it will go flat. Compare still (NOT MOVING).The joke fell flat - no one laughed.
Meaning of FLAT in English
Cambridge English vocab. Кембриджский английский словарь. 2012