What is the meaning of "flat-footed" in this context?

The word ‘flat-footed’ appears several times in the Harry Potter books and I am not quite sure what to make of it. It could be interpreted in several different ways, I think. And normally in the dictionaries, this word is an adjective but Merriam-Webster’s Dictionary denotes it could be an adverb too and the meaning, in that case, can be quite different from when the word is an adjective.

Let me quote two sentences where this word appears in the book first.

<1. “What — the — devil — are — you — doing?” said Uncle Vernon through gritted teeth, his face horribly close to Harry’s. “You’ve just ruined the punch line of my Japanese golfer joke. . . . One more sound and you’ll wish you’d never been born, boy!”
He stomped flat-footed from the room.
Shaking, Harry let Dobby out of the closet.>

<2. But as Filch lowered his quill, there was a great BANG! on the ceiling of the office, which made the oil lamp rattle.
“PEEVES!” Filch roared, flinging down his quill in a transport of rage. “I’ll have you this time, I’ll have you!”
And without a backward glance at Harry, Filch ran flat-footed from the office, Mrs. Norris streaking alongside him.>

And Merriam-Webster’s Dictionary says the following.

flat-footed adjective
Definition of flat-footed (Entry 1 of 2)
1: affected with flatfoot
broadly: walking with a dragging or shambling gait
2a: firm and well balanced on the feet
b: free from reservation: FORTHRIGHT
had an honest flat-footed way of saying a thing
3: not ready: UNPREPARED —used chiefly in the phrase catch one flat-footed
4: proceeding in a plodding or unimaginative way: PEDESTRIAN
flat-footed prose

flat-footed adverb
Definition of flat-footed (Entry 2 of 2)
1: in an open and determined manner: FLATLY
2: with the feet flat on a surface (such as the ground)

What definition do you think should be applied to the word in this case?