腳真的是平的
這一族就是畫面本身，講的是身體的事實，沒有比喻。
adjective · B2
Having feet whose soles rest flat on the ground, because the curve under the middle of the foot is low or is not there at all. It is used about people and also about animals such as bears.
扁平足的
核心畫面
Picture a foot pressed completely down: heel, middle and toes all touching the floor at once, with no gap under the arch. A foot like that is steady, but it has nothing stored in it. To move fast you have to lift a heel first and push off, and a foot that is already all the way down has nowhere to push from. Every meaning of this word comes out of that one position — first as the real shape of a foot, then as the feeling of being stuck to the ground at the moment something happens.
可以想成一隻完全踩平的腳：腳跟、腳心、腳趾同時貼著地板，中間沒有空隙。這樣站很穩，可是身上沒有存著任何彈力。要快速移…更多收起
可以想成一隻完全踩平的腳：腳跟、腳心、腳趾同時貼著地板，中間沒有空隙。這樣站很穩，可是身上沒有存著任何彈力。要快速移動，得先把腳跟抬起來再蹬出去。而一隻已經整個踩到底的腳，沒有可以蹬的地方。這個字的每個意思都從這個姿勢長出來。先是腳真實的形狀。再來是「事情發生的那一刻，人還黏在地上」的那種感覺。From the word’s history
The word is simply flat plus footed, 'having feet of a flat kind'. Nothing is hidden in it: the compound says the picture out loud, and the later meanings are that picture used about movement and about handling things.
這個字就是 flat（平的）加上 footed（有某種腳的），字面是「腳是平的那一種」。它沒有藏什麼典故：複合字本身就把畫面講出來了，後面的意思只是把同一個畫面拿去講動作和做事的方式。
Wiktionary, entry “flat-footed”, etymology 1: “From flat (adjective) + footed (adjective).” (packet source_ref kaikki:3f6f69a190036ae4:1519726238)
Three branches
腳跟踩死，就彈不起來
把注意力從「腳長什麼樣」移到「這樣站能不能動」，就得到這一族。這是意思上的連結，不是說歷史上先後的來源。
adjective · B2
Moving in a heavy, slow, awkward way, as if your heels are stuck to the ground and you cannot turn, jump or start running quickly.
動作笨重的；沒有彈性的
事情來的時候，腳跟還在地上
這一族把同一個姿勢定格在「事情突然發生」的那一秒，重點從動作本身變成時機。
adjective · C1
To surprise someone at a moment when they are not ready, so that they cannot answer or act as well as they normally would. It is usually heard in American English, and very often in the passive: be caught flat-footed.
打得措手不及