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.
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)
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