But aren't all feet more or less flat? You may be asking yourself.
Well, they are on top, if you're human, but the essential thing about flat feet is that they're flat underneath. Most of us have an arch between the ball of the foot and the heel, and the flat-footed don't.
Flat-feet used to be a reason for being rejected for military service, and it's true that the condition can cause knee and other problems, but usually people with flat-feet carry on just as normal and have no problems at all.
However, a flat-footed gait is said to be pondorous and heavy, and so anyone who puts their foot in it - in the sense of being outstandingly tactless - is often deemed called flat-footed. A flattie, short for flat-foot, is a British slang word for those other traditionally pondorously heavy-stepping characters, policemen.
As an extension of this meaning, to be flat-footed is to be clumsy or awkward, or perhaps simply to be not quite up to date with what's going on, as in catching the flat-footed.
Now, no one would want to be any of those things, so it's twinkle-toes time today for me.
I think that a simple pirouette at the fish counter of the supermarket will probably do.
Thing Not To Be Today: flat-footed. This word has meant flat more or less forever. The Greek form of the word was platus.