From Longman Dictionary of Contemporary Englishflat feetˌflat ˈfeet noun [plural] MIa medical condition in which someone’s feet rest flat on the ground because the middle of each foot is not as curved as it should be
Examples from the Corpus
flat feet• Fagg fitted in: his eyesight was very poor and he had flat feet.• He strutted about the office on his flat feet and smiled too much.• Carla Sloane was out on the lane, her huge flat feet surrounded by cases, umbrella and bags.• At school they had once heard of some animal which had become extinct because of its huge flat feet.• I thought your flat feet were firmly on the ground and your grubby little fingers always ready to grab the golden egg. 