[1761+; the form flat down as pancakes is found by 1611]
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with flat as a pancake
Extremely level, especially too much so. For example, There are no hills; this terrain is flat as a pancake. This simile dates from the 1500s and has survived its contemporary, flat as a flounder. It is sometimes used, either disparagingly or ruefully, to describe a small-breasted woman.