Wiktionary
a. (context simile English) Particularly or extremely flat.
Wikipedia
Flat as a Pancake was the first studio release of the soft rock, pop rock band Head East. At first, the album was released independently on Pyramid Records. However, when radio stations began to play the song " Never Been Any Reason," A&M Records signed the band and re-released the album in June 1975.
It is Head East's only album to be certified Gold by the RIAA.
Head East recorded the album at Golden Voice Studios in South Pekin, Illinois.
The diner photographed on the back cover of the A&M reissue is the Rite-Way Diner, still in business (now as the Olivette Diner) at 9638 Olive Blvd, Olivette, Missouri.
Usage examples of "flat as a pancake".
Most of it is flat as a pancake and as interesting as cold pancakes without butter or syrup.
The whole area was flat as a pancake, the rain gleaming off the slick black tarmac of the new two-acre car park destined to house the vehicles of those who would one day work in the intended office blocks nearby.
The crest of the summit was rock, all right, but it was as flat as a pancake.
They just lie there, flat as a pancake, no interest, no initiative.
I don't blame them: he would have squashed them flat as a pancake if he fell!
The Mob didn't slow until they'd reached the finish line, and behind them the course was flat as a pancake.
All around it cars got dented and torched and smushed flat as a pancake, but nothing ever happened to Big Blue.