"flapjack, griddle-cake, thin cake of batter fried or baked in a pan," c. 1400, panne-cake (late 13c. as a surname), from pan (n.) + cake (n.); as symbol of flatness c. 1600 (Middle English had as plat a kake, early 15c.). Colloquial Pancake Tuesday for "Shrove Tuesday" is from the old custom of eating them then.
"to squeeze flat," 1879, from pancake (n.). Later, of aircraft, "to fall flat" (1911), with figurative extension. Related: Pancaked; pancaking.