Learn English – origin of “5 seconds flat”

etymologyidioms

Does anyone know the etymology of this expression?

"He ran down the street in five seconds flat"

I found this explanation of meaning at Wordreference but would like to know where the expression comes from:

"In seconds flat" is an uncommon way of saying in no more than a few
seconds.

What kind of word is flat in this instance? Is this a UK expression or used worldwide?

Sorry, I know that's 3 questions, but they're all connected.

Best Answer

Flat has been used as an adverb to mean ‘exactly’ since the sixteenth century. The OED records its use in the context of various kinds of measurement as being of US origin. The earliest citation in this sense is from ‘Webster’s New International Dictionary of the English Language’ in 1909.