1. (adj) Really fast. 2. (adv) In the manner of someone putting everything she's got into whatever she's doing. Example: 1. You can take this corner flat biscuit and she won't lift. 2. He's really going flat biscuit there.
FLAT BISCUIT
Meaning of FLAT BISCUIT in English
Slang English vocab. Английский сленговый словарь. 2012