flat strap

Adverb

 * 1)  At maximum capability, intensity, or speed.
 * We had the car going flat strap down the highway when the cops pulled us over.

Adjective

 * 1)  Extremely busy.
 * Can you call me back tomorrow, mate? I′m flat strap at the moment.

Usage notes
"Flat chat" is commonly misused in place of "flat strap", however the origin of the word - from its usage in horse riding - points to "flat strap" being the correct implementation.